It’s one thing to know what something is, and another thing to know whether it exists or not; because we can know what something is without at the same time having some sense perception of it, but not whether it is. In fact, in all that comes to be by art or mind it is necessary that the definition of something be known before the thing defined exists in reality; this is what allows us to constuct triangles, or cars, or to test an hypothesis about the existence of a tenth planet or the luminiferous aether. The definition of the thing or “what it is” is a necessary principle in causing the thing to exist, and/or knowing whether it exists or not.
All this means that the definition of something stands indifferently to existence: a thing defined or known either can exist or cannot. In Aristotle’s terms, “what a thing is” stands to existence as potency to act.
Picture thinking, when confronted with the word “is”, finds itself unable to make any distinction between the “is” that expresses what something is, and the “is” that expresses whether it is. This is all a logical consequence of the inablity of picture thinking to grasp passive potency. Hume gives a good example of this:
It is far from being true, that in every judgment which we form we unite two different ideas, since in that proposition, God is, or indeed, any other, which regards existence, the idea of existence is no distinct idea, which we unite with that of the object (Treatise, Book I, sec. vii.)
The locus classicus of this “existence is no distinct idea” is from Hume’s disciple Kant, who bases his whole refutation of the proofs for the existence of God on the claim that “One hundred possible thalers contain not one coin more than one hundred actual thalers”. Both these statements are just a consequence of their picture thinking. It is true that when one imagines a man, and then imagines him to exist, nothing changes about the image. This is because an image as such must show something as actual; for it has no access to the potential. Existence therefore becomes something already present in everything and/or nothing at all- which both constitute a philosophical regression.