Synthesized answer
"Platonic Ideas" are distinct from the "ideas" discussed in Book I in that Platonic Ideas depend on the union of imagination and reason, and are the principal subject of the third book [1]. The "ideas" in Book I, specifically "ideas of perception," comprehend the whole visible world and the conditions of its possibility [5]. The passages indicate that "ideas" in Book I are considered from the side of knowability, where presented objects are regarded as ideas, abstracting from will [2, 3].
The passages also distinguish between "ideas of perception" and "abstract ideas," with abstract ideas forming a class called concepts, which are exclusively possessed by humans and are related to reason [5]. This suggests that "Platonic Ideas," requiring both imagination and reason, might encompass or relate to these abstract concepts, differentiating them from the broader category of "ideas of perception" that constitute the empirical world [1, 5].
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
ct ideas which cannot be presented in perception, but are general, and have no individual existence in space and time. Only in single cases do we pass from the conception to the perception, do we construct images as _representatives of concepts_ in perception, to which, however, they are never adequate. These cases are fully discussed in the essay on the principle of sufficient reason, § 28, and therefore I shall not repeat my explanation here. It may be compared, however, with what is said by Hume in the twelfth of his “Philosophical Essays,” p. 244, and by Herder in the…
adopted in consequence of some arbitrary abstraction. And yet it is a conception from which he can never free himself. The defectiveness of this view will be corrected in the next book by means of a truth which is not so immediately certain as that from which we start here; a truth at which we can arrive only by deeper research and more severe abstraction, by the separation of what is different and the union of what is identical. This truth, which must be very serious and impressive if not awful to every one, is that a man can also say and must say, “the world is my will.” In this…
t by Sir William Jones in the last of his essays: “On the philosophy of the Asiatics” (Asiatic Researches, vol. iv. p. 164), where he says, “The fundamental tenet of the Vedanta school consisted not in denying the existence of matter, that is, of solidity, impenetrability, and extended figure (to deny which would be lunacy), but in correcting the popular notion of it, and in contending that it has no essence independent of mental perception; that existence and perceptibility are convertible terms.” These words adequately express the compatibility of empirical reality and…
er is nothing but causality; the concept (as will appear immediately) is nothing but relation to a ground of knowledge. This thorough and consistent relativity of the world as idea, both according to its universal form (subject and object), and according to the form which is subordinate to this (the principle of sufficient reason) warns us, as we said before, to seek the inner nature of the world in an aspect of it which is _quite different and quite distinct from the idea_; and in the next book we shall find this in a fact which is just as immediate to every living being as the idea.…
is known and present to the reader, for if it had not been already said it would necessarily find its place here. § 3. The chief distinction among our ideas is that between ideas of perception and abstract ideas. The latter form just one class of ideas, namely concepts, and these are the possession of man alone of all creatures upon earth. The capacity for these, which distinguishes him from all the lower animals, has always been called reason.(5) We shall consider these abstract ideas by themselves later, but, in the first place, we shall speak exclusively of the _ideas of…