Descartes’ Three Substances: God, Thought, and Extension
It is important to mention that applying the first principle of the method has resulted in the first truth. From this, Descartes wants to infer the existence of extra-mental reality, of material things. He does this through his theory of ideas, which are objects of thought, intermediaries between thinking and thought. Descartes discusses the nature of ideas and realizes that some have more reality than others. He distinguishes between adventitious ideas, fictitious ideas, and innate ideas. These last are the only ones that can demonstrate extra-mental existence since their existence can be inferred from objective to subjective existence as an idea.
He discovers the idea of perfection and infinity, which he identifies with the idea of God. Then, from the very idea of God, he demonstrates the existence of God, giving four arguments:
- The argument applied to the idea of an infinite Being
- The argument of God as the cause of my being
- The ontological argument
God must exist because “absence” is less perfect than “existence,” and God is infinite, eternal, perfect, immutable, and the guarantee of truth. There may be misleading because it would be an imperfection, so he shows that the ideas presented are clear and distinct truths. Thus, the cause of the error can only be us, imperfect beings that we take for granted, and different ideas that are confusing. The first rule of method and criterion of truth only has validity by the existence of God.
Having demonstrated the existence of God, Descartes wants to prove the existence of the world, saying that God cannot allow ourselves to be deceived into thinking that the world exists. So our senses are not seeking to deceive us but are misinterpreted by precipitation or prevention, the data provided to us. Descartes has discovered three realms of reality:
- God or infinite substance
- The self or thinking thing
- Material things or the corporeal
Infinite substance is applicable only to God; He does not need more than Himself to exist. The res cogitans and res extensa are substances that do not need more than God to exist. To know of their existence and to know it, it is necessary to charge the same attribute. Attributes are qualities that cannot exist by themselves, and each substance has an attribute that is its own nature. Knowing that attribute is necessary to know the substance.
The attribute of the “thinking self” is thought; it appears to us clearly and distinctly as its own soul. In the case of material things, its attribute is extension. Also, to see if all the qualities we perceive in things have objective reality, he differentiates between primary and secondary qualities. The only thing that has objective reality in the body is what we perceive clearly and distinctly; these characteristics have only primary qualities, which can be expressed mathematically, for example, extension, volume, movement, and figures. Descartes limits real-world knowledge to primary qualities.
Secondary qualities such as odor, heat, or sound do not exist objectively in things; they are subjective assessments and have the function of teaching what we need and what is harmful, but they do not teach us anything about the truth of things. The latter is typical of reason and the senses.