For J. Greco, Reid in his critique of the Humean and Berkelyan skepticism about the knowledge of the external world uses as a central point the criticism and rejection of the theory of ideas (here the term “idea” means “subjective modification”), both with regard to species For Greco, Reid’s argument would be valid only for one of the two skeptical arguments and specifically the one concerning the “inconceivability of the external world”. For the other, that is, the evidence argument would be worth rather a sound theory of According to Greco, Reid is right about the consequentiality of skepticism starting from the theory of ideas, at least as understood by Reid himself, by Hume and by Berkeley. However, Reid is wrong both in believing that this theory is sufficient to refute any argument. skeptical both in believing that it is common to all previous philosophers (ancient and modern).

Regarding this last point, Greco does not seem to completely correct Reid, admitted I intend, for example, that, in scholasticism and in Thomism in particular, the theory of concepts (= ideas) but in general that of cognitive species is quite different.

According to Reid, the commonly accepted theory of ideas would be: our impressions (mental and / or sensitive images) are the immediate and internal objects to our thought. External objects would be objects of remote knowledge to be reached through inference. We can immediately ask ourselves a question: is this theory really commonly accepted by every philosopher?

In the Thomist tradition, impressions are not the immediate object of knowledge but that through which we conceive things: they are the result of the process of apprehension not themselves an object of immediate apprehension. They are not what we know immediately but the conceived images of known realities. Such images, the word itself says, have full resemblance but not really identity with the things (the res) of which they are images. Reid on his part, however, does not deny the existence of ideas (intelligible species) or impressions in general (intelligible and / or sensitive species). Reid denies that they are the immediate known object. And on this, I believe, there is agreement with the realistic conceptualism of Thomism. Reid also conceives what we have called species as the result of the process of ideation. For Greco, who tries in a certain sense to justify Reid’s historical-hermeneutic position, this problem does not arise at all. Greco would probably reply that it is a matter of grasping the theoretical core of the Scottish philosopher’s anti-skeptical argument. So let’s move on to this discussion.

The main skeptical argument examined by Reid is the following, at least in the formulation given by the Greek himself: 1) Only ideas and sensations are the immediate objects of thought; 2) all the rest must be conceived for the mediation of our impressions; 3) also the external objects to which our impressions would resemble; 4) no impression (idea and / or sensation) resembles an external object; Therefore 5) it is not possible to conceive of any external object. Given the theory of ideas thus reported, the inconceivability of the external world seems consequential. Greco states that Reid rejects all premises except (4). However, we can make some observations. If one accepts (1), one must accept that thought starts from itself, that is, that one should conceive before conceiving anything; (2) it is not obvious; (3) can easily be stated in reverse without contradiction. (4) instead it is incorrect because in order to know if an intelligible and / or sensitive species resembles or does not resemble an (external) object I have to know the species and the external object and therefore I should conceive the same object of which the skeptic claims to demonstrate its inconceivability with this same premise. To admit (4) means to contradict then (1) and (2) above all. We said that Greco states that Reid only accepts (4). Is it because Reid is more focused on denying the epistemic immediacy of ideas? Greco would answer yes. However, logically it seems to me that denying (4) makes a system with the same negation as the other premises. To affirm that mental images do not resemble objects seems to presuppose instead that we know these objects, and with immediacy. After all, doesn’t it seem free to identify “similarity” with “identity”? Furthermore, how can one, by accepting (1) speaking coherently about ideas and / or impressions in general, since one would accept that there is no possibility of having a real referent for the same ideas, which means that the images do not have the possibility itself of having meaning? Perhaps the theory of ideas criticized by Reid can be said to empty itself from within. Having made these considerations, Reid can always affirm that he is right in arguing and that there is an external object of our thought and that ideas are the result of the apprehensive operation not the immediate object. Greco on his part would admit, as in fact he does, that Reid’s critique of the theory of ideas is valid for a single skeptical argument but not sufficient for all. Perhaps we should see the link between the Scottish philosopher’s critique and the theory of evidence itself. Greco states that this link is missing from the evidence argument: there would be no trace of the theory of ideas.

However, Reid and Greco would always be right in arguing that given the theory of ideas the very inference of the external world would be inconceivable. Therefore this theory must be rejected. On the second argument, Greco believes that Reid’s criticism is insufficient. Adequate evidence should be provided for immediate knowledge of external objects. But how can we fail to see that in Reid’s discussion of the theory of ideas is a procedure for reductio ad absurdum of the same anti-evidence claims of Humean and Berkeleyan skepticism? Greco would reply that the premise (3) of the evidence argument (an adequate inference is needed from the knowledge of our sensations to the knowledge of external objects) does not presuppose the theory of ideas. In my humble opinion, on the other hand, it actually presupposes that ideas are the immediate object of knowledge. Greco replies that they should not be understood as epistemically immediate. According to Reid, for Greco we should speak of a foundationalism that we can well define pluralistic, where there are different sources of immediate knowledge and where ideas are signs of things whose perception Greco defines as conceptually mediated but epistemically immediate. However, if the theory of ideas is still accepted, defining impressions as signs can be slippery ground. Unless it is distinguished as in scholasticism between signum formal and signum instrumentale. The theory of ideas criticized by Reid would consist in considering cognitive species as signs-things (for example, smoke which means a fire, or a flag which indicates nationality: first they must be known in themselves and then they make the other known, but they also presuppose that one already knows the other in order to then be able to make the association) and not as pure signs that are resolved all and totally in meaning. If we understand the species in the first way, it is clear that then we must admit that we must pass into the existence of what they are a sign of through an inference. But we must instead admit that impressions, sensitive and intellectual, are formal signs, otherwise we would fall into an infinite regress in the very conception of concepts and in the very impression of impressions, in a self-referential way. This also explains the immediacy of a certain cognitive moment which is apprehension.

Mario Padovano, op