The Philosophical Problem of Knowledge and Truth

Posted on Nov 21, 2025 in Philosophy and ethics

The Problem of Knowledge

“What is Truth?”, “Is knowledge possible?” and “If knowledge is possible, what is its origin?” are pervasive philosophical questions. Truth has been understood mainly as a correspondence with facts derived from sensation (as synthetic truth) or as a coherence between statements originating in reason (as analytical truth). However, some philosophers have defined truth in many other ways, and some of them have even rejected that truth really exists and that knowledge is possible. Let’s investigate all those different paths and their problems.

Empiricism: Knowledge from Experience

In the 4th century BCE, Aristotle (384–322 BCE), in his treatise On the Soul, defended that the soul (mind) is a tabula rasa (a blank slate), meaning all its contents originate externally. Individuals are born without built-in mental content (innate ideas), and their knowledge comes from experience and perception:

“What the mind thinks must be in it in the same sense as letters are on a tablet which bears no actual writing; this is just what happens in the case of the mind.”

Since the origin of the Latin word experientia is the Greek empeiria, the philosophers who defend that knowledge is based on experience are called empiricists or natural foundationalists. They defend the view that our system of belief needs some kind of foundation, and that it lies on the natural trust we have in our senses. “The veracity (truthfulness) of our senses and reasonings is itself part of the foundation. It cannot itself be demonstrated by standing on some other ‘original principle.’ For all of us, outside the philosophical study, it comes naturally to trust our common experience. […] The emphasis on natural ways of forming belief chimes in with another strand in (…) empiricists, which is their distrust of the power of unaided reason. For these philosophers, the best contact between mind and the world is not the point at which a mathematical proof crystallizes, but the point at which you see and touch a familiar object. Their paradigm was knowledge by sense experience rather than by reason.” (T)

Empiricism is then characterized by:

Emphasis on the role of evidence and sensory experience in the formation of ideas within our minds, and
A theory of truth as correspondence between mind and world.

We owe the first definition of truth as correspondence once again to Aristotle, who establishes in his Metaphysics that: “To say of what is that it is not, or of what is not that it is, is false, while to say of what is that it is, and of what is not that it is not, is true.” Following this definition, truth is achieved when there is a correspondence between what is thought or said (language) and reality. In the 20th century, Alfred Tarski set out a mathematical definition of truth for formal languages which can be seen as a correspondence theory of truth, since truth is defined as material adequacy (that is, adequacy between language and content, words and objects):

“p” is true if and only if p

“p” (with quotation marks) alludes to an expression within a language, and p (with no quotation marks) alludes to a fact. Hence, it is necessary and sufficient for an expression in a language to be true that it has a reference in reality (that the content of the expression exists materially). Therefore, if there is a correspondence between a belief or an assertion and facts, then they are true; if there is no such correspondence, they are false. The question is then how to establish this correspondence, and this apparently obvious theory of truth faces two major challenges:

The first one is the problem of sense data, which addresses questions like “What counts as ‘a fact’?”, “Do we have direct access to facts?” or “Are facts theory independent?”
The second one is the problem of induction, which addresses questions like “Can we reach necessary and universal knowledge from experience?”, “Is induction reliable?”, “Are synthetic truths possible?”

Maybe our mind is not as clean as it seems…

Rationalism: The Power of Pure Reason

Aristotle’s teacher, Plato (428–347 BCE), had a different theory of truth. For him, the mind was not a blank slate, and knowing was, in fact, remembering something already present in our minds but forgotten, hidden by our senses. For Plato, sensitive knowledge was misleading because facts were mere appearances, a delusive world of images imperfectly copying the authentic reality: the world of Ideas. If truth is necessary and universal, then it is beyond the physical world, in which there is nothing but contingency and particularism. Truth is understood as aletheia, that is, unveiling. Things are not as they seem to be; appearances deceive us and hide (as a veil) the true reality. We cannot trust our senses.

This mistrust of experience is characteristic of rational foundationalism or just rationalism. The model of knowledge for rationalists is not empirical sciences and induction, but formal sciences and the axiomatic-deductive method. The father of rationalism is René Descartes (1596–1650), who had a mathematical model of clarity in mind, a model in which the perception of truth is transparent, obvious, “clear and distinct” in Descartes’ words: either you see or you don’t, but true statements come into mind as obvious absolute truth whose falsity is unconceivable.

In his dialogue Meno, Plato proves this quality of mathematics as a privileged access to a truth beyond appearances. In this dialogue, Socrates demonstrates his method of questioning and recollection (maieutics) by interrogating a slave boy who works in Meno’s house. This slave is ignorant of geometry. The subsequent discussion shows the slave is capable of learning a complicated geometry problem. According to Plato, this proves that knowledge preexisted in the slave’s soul and that knowing is just recalling ideas from a past life in which the soul had direct access to universal and eternal ideas. Other rationalist philosophers (and to be perfectly strict, rationalism does not exist before Descartes, that is, before the seventeenth century) would not defend that ideas exist in a metaphysical world or that the soul preexists bodily life, but they would suggest in one way or another that there is something like innate ideas within the human mind. Knowing through reason looks like acknowledging, like finally understanding something which was already there but resisted coming to the surface. When a mathematical theorem is proved “you can just see that the theorem has to hold. This may come as a ‘flash’: a blinding certainty, or insight into this particular piece of geometrical truth. [A] geometrical example of a procedure [like Socrates’ with the slave] can make you ‘see’ something that you might only dimly have grasped. But if only we could see the rest of reality, mind, body, God, freedom, human life, with the same rush of clarity and understanding! Well, one philosophical ideal is that we can. This is the ideal of rationalism: the power of pure unaided reason. For the rationalist can see from her armchair that things must be one way and cannot be other ways, like [the diagonal of the square]. Knowledge achieved by this kind of rational insight is known as ‘a priori’: it can be seen to be true immediately, without any experience of the way of the world.” (T)

Those a priori ideas are to be understood either as innate ideas or just self-evident ideas which need no further proof because their falsity is unconceivable or leads to a contradiction. Sometimes we just know what’s true at a glance, by intuition.

Rationalism is then characterized by:

Emphasis on the role of intuition and deductive reasoning in the formation of ideas within our minds, and
A theory of truth as evidence (direct evidence of self-evident or innate ideas on one hand, and indirect evidence of derived ideas on the other hand).

The theory of truth as evidence (where evidence does not mean factual evidence, but intuitive or internal evidence) results from the application of the three principles of knowledge:

Principle of Identity: According to G. W. Leibniz, the law of Identity, which he expresses as “Everything is what it is,” is the first primitive truth of reason which is affirmative. Its symbolic notation would be “(A=A)”.
Principle of Non-Contradiction: According to Aristotle, the law of non-contradiction says that one thing cannot be and not be at the same time and in the same sense; therefore, truth is not self-contradictory. The symbolic notation of the principle would be “¬(A˄¬A)”.
Principle of Excluded Middle: Again, Aristotle says that a statement is either true or false and there is not a third possibility. Knowledge is then bivalent (the two only possible truth values are truth and falsity), and that’s why this principle can also be called the principle of bivalence, whose symbolic notation is “A˅¬A”.

However, those principles are mere formal rules; they guarantee that truth is transmitted from one statement to another. But where can we get the first truths from which we could deduce the rest of our system of knowledge? With an inner perception called intuition, by which we recognize self-evident truths, unproved statements whose falsity is untenable, like that of Euclid’s axioms. The truth of some analytic statements is perceived so “clear and distinct” that its denial would be absurd, that is, a contradiction. Therefore, rationalist philosophers hope to prove that knowledge can be shown to be analytic by careful “conceptual analysis,” that is, deriving any possible true statement by deduction and logic from a series of basic analytical truths, knowable by knowing the meaning of its constituents.

This notion of truth has to deal with several problems, but all of them are related to the topic of circularity: “Aren’t we defining the principles of knowledge in terms of intuitive evidence, and evidence as accordance with the principles of knowledge?”, “Does anything warrant that reality is logical or evident?”, “Can reason alone tell us something about the material world?”, “Is there just one possible logical model?” Maybe there’s nothing beyond appearances, and reason leads us to a fictional world…

Idealism and the Kantian Synthesis

Immanuel Kant (1724–1804) sought to reconcile the rationalist and empiricist traditions in epistemology. He did so in response to the skepticism of David Hume (1711–1776), whom Kant said had “awakened him from his dogmatic slumbers.” This synthesis of rationalism and empiricism is summarized in the following statement:

“Concepts without perceptions are empty, perceptions without concepts are blind.”

Hence, Kant agreed with the empiricists that “Concepts without perceptions are empty…”. Concepts (ideas) alone cannot constitute knowledge. Innate ideas do not constitute knowledge. There must be experience(s) for there to be knowledge. However, Kant also agreed with the rationalists that “…perceptions without concepts are blind.” Merely having experiences (perceptions) also does not constitute knowledge. There must be some way in which the mind organizes experience for there to be knowledge.

Kant also held that “It is true that all knowledge begins with experience…”. Without experience, no knowledge takes place. Experience is the ‘initiator’ of coming to possess knowledge. Here he once again agrees with the empiricist affirmation of the necessity of experience in knowing. However, Kant also held that “…it is not true that all knowledge arises from experience.” Experience is not the sole source of knowledge. There must also be ‘categories’ by which experience is structured and understood for there to be knowledge. Here he agrees with the rationalists. In short, for Kant, sensibility delivers the content of knowledge, its matter (what is known), whereas understanding gives form to that content, which would otherwise be meaningless—a rough collection of chaotic independent sensations just ordered in space and time by sensibility.

What constitutes Kant’s idealism is that he affirmed that the mind is “active” in the knowing process. The mind makes an active contribution to “the-world-as-it-is-known.” The “world-as-it-is-known,” (what Kant would call the ‘phenomenal’ realm), conforms to the mind in the knowledge process, rather than the mind conforming to a world that has its own mind-independent structure. Thus, knowledge has a profoundly subjective dimension (the mind) as well as an objective dimension (“the world-as-it-is-in-itself,”—what Kant called the ‘noumenal’ realm.) The mind contains ‘categories of the understanding.’ These categories are the ways in which the active mind forms or structures experience. For Kant, there were twelve such categories, one of which was the category of ‘causation.’ The active mind relates and understands experiences in terms of some event-experiences being causes, with other event-experiences being their effects. The basis of scientific knowledge is “every event has a cause.” Hume demonstrated that we cannot claim to know this on the basis of empiricist assumptions. He held that ‘cause’ is not a fact “out there” in the world. Thus, we can have no idea or knowledge of causal connections in reality; causal attributions are merely a type of mental ‘habit.’ Kant held that “every event has a cause” is a universally and necessarily true statement, something that we do know. It is not a mere mental ‘habit.’ But the truth of the statement is grounded in the ‘cause-category’ within the mind that actively structures, universally and necessarily, all our experience of the world. Kant assumed that every human being, as a rational being, possesses and utilizes the same categories of understanding. As reason is the same in all rational beings, so in principle the proper exercise of reason will lead any and all persons to knowledge claims that are objectively and universally valid. It follows that we do not know reality as it might be “in itself”—apart from how our minds structure experience of ‘mind-independent reality.’ We do not know ‘noumena.’ We only know reality in terms of how our active minds structure our experiences of mind-independent reality. We only know ‘phenomena.’

Deflationary Theories of Truth

Pragmatism: Truth as Utility

Quine recognized that when we encounter an experience which falsifies part of our web of beliefs, we are not usually at a loss to decide which of our beliefs to revise in response to it, but he claimed that this is simply because we are strongly disposed as a matter of fundamental psychology to prefer whatever revision requires the most minimal mutilation of the existing web of beliefs. But what justifies this criterion?

William James (1842–1910) defended that “True ideas are those that we can assimilate, validate, corroborate and verify. False ideas are those that we cannot.” Therefore, truth is not something static but dynamic, and in the process of validation, we are guided by a utility criterion: “the possession of true thoughts means everywhere the possession of invaluable instruments of action.” Truth is then what is useful. “True ideas would never have been singled out as such, would never have acquired a class-name, least of all a name suggesting value, unless they had been useful from the outset.” When lost in the woods and finding what looks like a cow-path, what makes true the statement “there must be human habitation at the end of the path”? The fact that I save myself if it is true. Why do we believe in modern biology or physics? Could we prove the theories on which they rely? No, but we are healed by the medicine resulting from our biological knowledge and transported by the dynamics resulting from our knowledge about physics. True is that which functions; success is the ultimate criterion for scientific knowledge.

Our scientific knowledge counts on the truth of pragmatic principles like Ockham’s razor (the principle of economy): among competing hypotheses, the one with the fewest assumptions should be selected (more complicated solutions may ultimately prove correct, but in the absence of certainty, the simplest theory should be selected). Even aesthetics plays a part in the competition of theories: all other things being equal, the more elegant theory would be chosen. But ultimately, who applies those criteria? Another salient pragmatist, Charles Sanders Peirce (1839–1914), claimed that truth is equivalent to the experts’ assent:

“The opinion which is fated to be ultimately agreed to by all who investigate, is what we mean by the truth, and the object represented in this opinion is the real. That is the way I would explain reality.”

Truth is useful, but can we say that what is useful is true? According to James, we can:

“The true is the name of whatever proves itself to be good in the way of belief, and good, too, for definite assignable reasons.”
“Any idea upon which we can ride …; any idea that will carry us prosperously from any one part of our experience to any other part, linking things satisfactorily, working securely, saving labor; is true for just so much, true in so far forth, true instrumentally.”

This suggests that beliefs are made true by the fact that they enable us to make accurate predictions about the future run of experience, or that they are valuable for life: a belief can be made true by the fact that holding it contributes to our happiness and fulfillment. Bertrand Russell objected that James would be committed to the truth of “Santa Claus exists,” but this belief might clash with other vital benefits; therefore, it wouldn’t be true in a straightforward way. Anyway, apparently truth would depend on weighing pros and cons, and believing would be more election than assent.

Consensus Theory of Truth (Habermas)

One of the more important living philosophers, Jürgen Habermas (1929), has defended a theory of truth as consensus. A sentence is true when it may reach the assent of all others in a discourse. The idea of “discourse” in Habermas’ philosophy refers to an “ideal speech situation” which depends on the adhesion of members of the public sphere to certain implicit rules:

Every subject with the competence to speak and act is allowed to take part in a discourse.
a. Everyone is allowed to question any assertion whatever. b. Everyone is allowed to introduce any assertion whatever into the discourse. c. Everyone is allowed to express their attitudes, desires and needs without any hesitation.
No speaker may be prevented, by internal or external coercion, from exercising his rights as laid down in (1) and (2).

In those circumstances where participants are free of any nonrational coercion, the basis of the debate would be reason and evidence, and the motivation to obtain a rational consensus. Truth would then be determined by the demand of reaching a rational consensus: truth would be the outcome of this ideal rational consensus.

Falsificationism (Popper)

What makes a theory truly scientific and not a fancy science-looking substitute?

The problem of the demarcation of science was philosopher Karl Popper’s (1902–1994) main concern. According to Popper’s critical rationalism, the scientific method is not inductive: science does not try to confirm hypotheses, but it nevertheless learns through experience with a scientific method of trial and error, of conjectures and refutations, in which hypotheses may survive provisionally but not be verified. Therefore, the scientific method is a method of testing hypotheses, but through testing, science does not pretend to justify or confirm its hypotheses, but to refute them. In some way, science works deductively and just uses modus tollens: logic can only refute hypotheses, never confirm them. “We never know, we can just conjecture.”

What is, then, the final criterion of demarcation of science? The falsifiability of its laws and theories. A theory should be considered scientific if and only if it is falsifiable: “statements or systems of statements, in order to be ranked as scientific, must be capable of conflicting with possible, or conceivable observations.” The laws and theories of pseudoscience cannot be falsified; there is no crucial experiment which would make them false because:

a) their laws and theories are ambiguous, so it is impossible to design a test which would clearly prove them false, or
b) their predictions are so general that almost everything would count as a confirmation of them; they are so vague that they are irrefutable, or
c) they use ad hoc hypotheses to confirm the theory after it has been falsified to render it immune to falsification.

Apparently, the more probable a theory is, the better it is, and if we have to choose between two theories which are equally strong in terms of their explanatory power, if one is more probable, we should choose it. Popper rejects this. Science should be interested in theories with a high informative content, because such theories possess a high predictive power and are consequently highly testable. But if this is true, paradoxically, the more improbable a theory is, the better it is scientifically, because the probability and informative content of a theory vary inversely (the more information a statement contains, the greater will be the number of ways in which it may turn out to be false). The theories which come closer to the truth are those with more informative content, which is in inverse proportion to probability but in direct proportion to testability.

Truth, then, can never be obtained, since knowledge seeks testability (falsification), and an absolute truth would be non-falsifiable (like a dogma). Therefore, all which is testable is fallible and could be falsified. Even observation statements, Popper maintains, are fallible, and science then is not a quest for certain knowledge, but an evolutionary process in which hypotheses or conjectures are imaginatively proposed and tested in order to explain facts or to solve problems. There is no final truth; truth is more a process.

But how are we to consider non-falsified theories? Corroborated theories? They are just better theories in terms of truth-likeness or verisimilitude. A ‘good’ scientific theory, Popper argues, has a higher level of verisimilitude than its rivals, that is, is closer to the truth than them. Thus, scientific progress involves the abandonment of partially true, but falsified, theories, for theories with a higher level of verisimilitude, i.e., which approach more closely to the truth. Scientific progress is then represented as progress towards the truth, and experimental corroboration is just an indicator of verisimilitude (not a proof of truth, since inductive truth would be unachievable because of theory-ladenness and the problem of induction).

Skepticism and Relativism

The Challenge of Skepticism

Consider some proposition, p. There are just three possible propositional attitudes one can have with regard to p’s truth when considering whether p is true. One can either assent to p, or assent to ¬p, or withhold assenting both to p and to ¬p. Now, consider this (meta) proposition concerning the scope of our knowledge, namely: We can have knowledge. Given that there are just three stances we can have toward any proposition when considering whether it is true, we can:

Assent that we can have knowledge.
Assent that we cannot have knowledge.
Withhold assent to both the proposition that we can have knowledge and withhold assent to the proposition that we cannot have knowledge.

The attitude depicted in 1 is that of a foundationalist (no matter empiricist or rationalist) and of philosophers defending some kind of deflationary theory of truth (truth is possible although fallible).

The attitude portrayed in 2 is a form of radical skepticism: knowledge is impossible. Its classical name is academic skepticism because the leaders of the Academy founded by Plato during the 3rd to 1st century BCE defended this view.

Finally, those assenting neither to the proposition that knowledge is possible nor to the proposition that knowledge is not possible can be called Pyrrhonian skeptics after Pyrrho (circa 365–275 BCE). The Pyrrhonians withheld assent to every non-evident proposition. That is, they withheld assent to all propositions about which genuine dispute was possible, and they took that class of propositions to include the (meta) proposition that we can have knowledge. For Pyrrhonians, both attitude 1 and 2 are dogmatic: I’m giving unjustified assent to the belief that knowledge is possible (1) and to the belief (the academic skeptic maintains confidence in the ability of reason to settle matters, at least with regard to the extent of our knowledge) that knowledge is impossible (2). So, according to Pyrrhonism, academic skepticism refutes itself; it is self-contradictory.

As Pyrrhonians, we are understanding skepticism as the suspension of assent to non-evident propositions, as a way of life in which certainties are absent. Non-evident propositions are those about which there can be legitimate disagreement. Pyrrhonians developed a method to recognize those non-evident propositions and practice the absence of assent: the modes. The modes are reminders of the relativity of perception (to refrain from assenting to judgments of perception) and reminders of discrepancy and relativity concerning reasoning, that is, reminders that the relevant object of inquiry is subject to legitimate dispute (to refrain from assenting to judgments of reason). Therefore, for skeptics, every possible belief is relative to subjects, species, societies… or improvable principles of knowledge (non-founded foundations), and against any possible argument to prove the truth of a proposition, it is possible to find a counter-argument to prove it false. We ought then to refrain from assenting to any non-evident proposition.

Nietzsche’s Perspectivism

According to Friedrich Nietzsche (1844–1900), an objective representation of reality is impossible because we do not have access to the objects of knowledge but to their image; therefore, language and knowledge refer to reality only through metaphors. Some of those metaphors are successful and become obvious images which fix the meaning of objects and prevail, eliminating all the other possible metaphors. Those “encysted” metaphors are finally called “real truth” or “the thing itself,” as if they were not metaphors, as if they were not just possible representations of the world but the world itself or the only possible perspective. When this happens, those metaphors are called “concepts.” But concepts falsify reality because they grant a privilege to a single perspective; concepts are rigid, they reduce plurality into unity (a concept refers to a plurality of similar things). Concepts impoverish reality; they control and describe it at the expense of simplifying it, of standardization, of dissolving its richness, its diversity.

Therefore, according to Nietzsche, we do not refer to reality, we create it. Science pretends to be objective, but there is no such thing as “objective facts”; any fact is always an interpretation. There are as many realities as perspectives; any metaphor has many interpretations and possible meanings. The truth of poetry is superior to the truth of science because it assumes that the depiction of facts is the expression of one or more perspectives, not the determination of the world itself. There are many truths, therefore none. There is no logic or sense underneath reality; there is no order underlying the apparent chaos, but we want to believe there is one just to feel more secure. Science and philosophy have mathematized reality; they have expelled from reality everything which was not static or quantitative, but reality is dynamic. In order to avoid facing the multiple perspectives of reality and truth, the scientist and the philosopher have granted privilege to the expression of just one perspective expressed through numbers to make intelligible a non-intelligible world, and they have called this perspective the actual reality, the reality itself, true beyond appearances.

Scientific Relativism (Kuhn)

In the sixties, there was a historical turn in philosophy of science. The study of the history of science suggested that most of the criteria of demarcation of science were myths and emphasized the human, subjective quality of scientific change. For thinkers like Thomas S. Kuhn (1922–1996) or Paul Feyerabend (1924–1994), the practice of science is unavoidably tied to historical and social components, and therefore contingent elements are crucial to scientific theories.

In The Structure of Scientific Revolutions, Kuhn defended new concepts to interpret science which raised doubts about the objectivity of scientific knowledge. According to history, for Kuhn, the ultimate criterion of scientific truth is being compatible or not with the dominant paradigm, and the final criterion of demarcation of science is belonging or not to the dominant paradigm. A paradigm designates the whole scientific community sharing the same “big theory” and the same research methodology in a historical moment. So, in periods of normal science (most of them), all experiments aim to confirm the pre-existent paradigm; therefore, the Popperian crucial experiments are irrelevant because the big theory is immune to experimental falsification: what is not adapted to the dominant paradigm is not considered; if it is considered, then it is hidden or just adapted to fit in the paradigm, and finally, observation is theory-laden (the paradigm creates its own experience; it just selects what is inside its scope). The progress of science is therefore non-accumulative but revolutionary: scientific revolutions imply paradigm shifts which imply a whole new conceptual scheme, a new worldview. Therefore, different paradigms and macro-theories are incommensurable; they cannot be compared or translated to each other because even though the same terms might be used, they have suffered a semantic displacement—they don’t have the same meaning anymore. Only within a paradigm are knowledge and experience possible. Falsificationism is naive because history proves that seldom, if ever, are theories abandoned after negative tests. Theories are permanently full of empirical anomalies, experiences which do not fit in the theory, inconsistent with it, which refute the theory. But those anomalies only become relevant within a scientific revolution, when there is an alternative paradigm which accounts for those anomalies. Truth is then relative to the dominant paradigm.

This account is an example of scientific relativism; nevertheless, the same scheme can be applied to society, culture, or individuals: truth would be relative to the dominant social, cultural, or subjective paradigm. But if this is the case, if there are as many truths (even incompatible and contradictory) as societies, cultures, or individuals, is there really something we may call true?