René Descartes was a typical example of a Renaissance intellectual: soldier, scientist, philosopher and speculative psychologist . He studied with the Jesuits, and his formation was both metaphysical and humanistic. His influence has been decisive for his reformulation of rationalism , and his inclusion in a mechanistic system.

Descartes (1596-1650) and Rationalism

Just as the scepticism of the Sophists was responded to with Plato’s rationalism, Descartes’ rationalism was a response to the humanist scepticism of the previous period which, having placed man at the centre of the world, did not trust his own strength to sustain him.

Descartes did not accept the belief of the sceptics in the impossibility of knowledge , nor in the weakness of reason. He decided to systematically doubt everything until he found something that was so clearly true that it could not be doubted . Descartes discovered that he could doubt the existence of God, the validity of sensations (empiricist axiom), and even the existence of his body.

Cogito ergo sum: the first and undoubted truth

He continued on this path, until he discovered that he could not doubt one thing: his own existence as a self-conscious and thinking being. One cannot doubt that one doubts, because, in doing so, one performs the very action that one denies. Descartes expressed his first undoubted truth with the famous: Cogito ergo sum . I think, therefore I am .

From his own existence, Descartes justified the existence of God through arguments that were already questioned at that time. He also established the existence of the world and of his own body, and the general accuracy of perception.

Descartes believed that a correct method of reasoning can discover and prove what is true. He advocates, as a good rationalist, the deductive method: discovering by reason the obvious truths and deducing from them the rest . This method is opposed to the inductive method proposed by Francis Bacon and adopted by empiricists.

Descartes, however, did not rule out the usefulness of the senses, although he thought that facts have little value until they are ordered by reason.

From Philosophy to Psychology and Knowledge about Cognition

Descartes was not the first to justify his own existence in mental activity. Already the first rationalist, Parmenides , had affirmed ” Because it is the same to think and to be “, and St. Augustine had written “if I deceive myself, I exist” (for Descartes, on the other hand, who doubts all transcendent Truth, the question would have been “if I deceive myself, I do not exist”), and only a century before, according to Gomez Pereira: “I know that I know something, and who knows exists. Then I exist. “The Cartesian novelty lies in sustaining all meaning over doubt, and in grounding the only certainty in logical truth.

Since Descartes, philosophy has become increasingly psychological , seeking to know the mind through introspection, until the emergence of psychology as an independent scientific discipline in the nineteenth century, based on the study of consciousness through the introspective method (although only for the first generation of psychologists).

Descartes affirms the existence of two types of innate ideas : on the one hand the main ideas, those that cannot be doubted, although they are potential ideas that require experience to be updated. But it also speaks of innate ideas regarding certain ways of thinking (what we would now call processes, without specific contents, only ways of operating: for example transitivity). This second kind of innatism will be developed in the 18th century by Kant , with his synthetic a priori judgments.

Universal Mechanism

Descartes enriched the theory of Galileo with principles and notions of mechanics, a science that had achieved spectacular successes (clocks, mechanical toys, fountains). But Descartes is also the first to consider mechanistic principles as universal, applicable to inert matter as well as living matter, to microscopic particles as well as to heavenly bodies.

Descartes’ mechanistic conception of the body is as follows: the characteristic of the body is that of being an extensive res, a material substance, as opposed to res cogitans or thinking substance.

These different substances interact through the pineal gland (the only part of the brain that does not repeat itself hemispherically), affecting each other mechanically.

The body has receiving organs and nerves or hollow tubes that communicate internally with each other. These tubes are crossed by a kind of filaments that at one end join with the receptors, and at the other end with some pores (as a cover) of the brain’s ventricles that when opened allow the “animal spirits” to flow through the nerves, which influence the muscles causing the movement. I did not therefore distinguish between sensory and motor nerves, but I had a rudimentary idea of the electrical phenomenon that underlies nerve activity.

The legacy of René Descartes in other thinkers

It will be Galvani , in 1790, who, from the verification that the contact of two different metals produces contractions in the muscle of a frog, demonstrates that electricity is capable of provoking in the human body a similar effect to that of the mysterious “animal spirits”, from which it could easily be deduced that the nerve impulse was of a bioelectric nature. Volta attributed this effect to electricity, and Galvani understood that it was generated by the contact of two metals; from the discussion between the two arose, in 1800, the discovery of the battery, which initiated the science of electric current.

Helmholtz , in 1850 thanks to the invention of the myograph, measured the reaction delay of the muscle when stimulated from different lengths (26 meters per second). The mechanism of the sodium pump would not be discovered until 1940.

The importance of the pineal gland

In the pineal gland Descartes places the point of contact between the spirit ( res cogitans , thinking substance) and the body , exercising a double function: control over the excessive movements (passions) and, above all, consciousness. Since Descartes did not distinguish between conscience and consciousness, he deduced that animals, which did not possess a soul, were like perfect machines without a psychological dimension, that is, without feelings or consciousness. Already Gómez Pereira had denied the psychological quality of sensation in animals, leaving their movements reduced to complicated mechanical responses of the nerves acted on from the brain.

The result was that a part of the soul, traditionally associated with movement, became an intelligible part of nature and therefore of science. Psychological behaviorism, which defines psychological behavior as movement, is indebted to Descartes’ mechanism. The psychism was configured, on the other hand, only as thought , a position that would reappear later with cognitive psychology, if the latter is defined as a science of thought. For Descartes, however, thought was indissociable from consciousness.

One characteristic, however, common to these approaches, as is widely the case in the rest of modern science, is the radical separation between the subject that knows and the object of knowledge. Both movement and thought will become automatic, proceeding according to predetermined causal chains in time.