"Nothing passes from external objects to the soul except certain motions of matter (_mouvemens corporels_), but neither these motions, nor the figures which they produce, are conceived by us as they exist in the sensory organs, as I have fully explained in my "Dioptrics"; whence it follows that even the ideas of motion and of figures are innate (_naturellement en nous_). And, _à fortiori_, the ideas of pain, of colours, of sounds, and of all similar things must be innate, in order that the mind may represent them to itself, on the occasion of certain motions of matter with which they have no resemblance."
Whoever denies what is, in fact, an inconceivable proposition, that sensations pass, as such, from the external world into the mind, must admit the conclusion here laid down by Descartes, that, strictly speaking, sensations, and _à fortiori_, all the other contents of the mind, are innate. Or, to state the matter in accordance with the views previously expounded, that they are products of the inherent properties of the thinking organ, in which they lie potentially, before they are called into existence by their appropriate causes.
But if all the contents of the mind are innate, what is meant by experience?
It is the conversion, by unknown causes, of these innate potentialities into actual existences. The organ of thought, prior to experience, may be compared to an untouched piano, in which it may be properly said that music is innate, inasmuch as its mechanism contains, potentially, so many octaves of musical notes. The unknown cause of sensation which Descartes calls the "je ne sais quoi dans les objets" or "choses telles qu'elles sont," and Kant the "Noumenon" or "Ding an sich," is represented by the musician; who, by touching the keys, converts the potentiality of the mechanism into actual sounds. A note so produced is the equivalent of a single experience.
All the melodies and harmonies that proceed from the piano depend upon the action of the musician upon the keys. There is no internal mechanism which, when certain keys are struck, gives rise to an accompaniment of which the musician is only indirectly the cause. According to Descartes, however--and this is what is generally fixed upon as the essence of his doctrine of innate ideas--the mind possesses such an internal mechanism, by which certain classes of thoughts are generated, on the occasion of certain experiences. Such thoughts are innate, just as sensations are innate; they are not copies of sensations, any more than sensations are copies of motions; they are invariably generated in the mind, when certain experiences arise in it, just as sensations are invariably generated when certain bodily motions take place; they are universal, inasmuch as they arise under the same conditions in all men; they are necessary, because their genesis under these conditions is invariable. These innate thoughts are what Descartes terms "vérités" or truths: that is beliefs--and his notions respecting them are plainly set forth in a passage of the _Principes_.