Descartes, Locke, and Hume all use the word "idea" in the most important parts of their most important writings. Their use of it is central to their philosophy.
For Descartes, it is his idea of a supreme being from which he infers that being's real existence, that is, God's existence "outside" Descartes own mind.
For Locke, a few simple ideas are those from which all our thoughts, imaginations, and creations are composed.
Hume asserted that, "When we entertain, therefore, any suspicion, that a philosophical term is employed without any meaning or idea (as is but too frequent), we need but enquire, from what impression is that supposed idea derived?" Hume, who, with such confidence in the truth of the thesis that all "ideas" are "copies" of "impressions," searched in vain for the original experience of the "idea of necessary connexion", that is, of cause-and-effect, from which our idea of it must have been "copied," and who, coming up empty-handed, therefore concluded that cause-and-effect does not really exist, apparently trusted himself enough to let the entirety of his intellectual health and philosophical reputation rest on his understanding of exactly what an "idea" is.
What is an idea, for these three men?
Hume is the writer I will focus on here, both because circumstances permit, and because he actually provides a definition to work with.
He says that ideas are "copies" of impressions.
Good.
There is a curious passage from Plato's Sophist, wherein the characters are discussing that amazing ability to create images and likenesses. The stranger from Elea, who is leading the conversation, asks Theaetetus, the young philosopher, a series of questions. In 240A7ff, (with some modification for the sake of clarity,) it goes like this:
"What is an image?"
"Well... what would we say an image is except, of course, another of the sort that has been made similar to simply the real thing."
"And by 'another of the sort' you mean what is simply the real thing, or why did you say 'of the sort'?"
"It's in no way simply the real thing, of course, but resembling."
"Meaning by 'simply the real thing' 'that which really is?"
"That's so."
"And what of this? Isn't whatever's not real contrary to real?"
"Why certainly."
"By the resembling, then, you mean, after all, that which is not really, if, that is, you'll say it's not simply the real thing."
"But it still is in a sense."
"Though not truly, at any rate, you assert."
"No indeed, but it is, in reality, a semblance."
"So, not really being, it really is that which we mean by a 'semblance.'"
"It's probably that 'what which is not has got woven in with 'that which is' in a kind of braid of this sort. It's very strange, too."
"Of course it's strange!" the stranger replies.
My question is essentially the same, and I address it to Mr. Hume: What is a "copy"? If you agree with the product of the conversation between Theatetus and the Eleatic stranger, you will say that the copy is a braid --or mixture -- of "that which is, and that which is not." If this is right, then the idea of solidity is a mixture of solidity and not-solidity. This is strange, but if it is true, then there is one thing we must attend to: The idea of solidity actually has at least some solidity itself in it. It is not "merely" an idea... that is, it is not entirely 'un-solid.' Though it is un-solid, partly. Very strange...
To turn to Descartes, I will ask him whether or not his mental copies, that is, ideas, are simple things or mixtures... if they are simple, what are they made of? Mind? If that is the case, how can mind resemble anything but mind?
Are they then made of the object itself? This does not seem right. If that is the case, then my idea of a computer simply is the computer itself, and my mind is the world around me; they are identical. All my ideas are things themselves, including my computer, my chair, and even other people. This does not seem right, for quite a few reasons.
Then are they mixtures of that which is and that which is not? If that is the case, then my idea of a computer and the computer itself are naturally connected. It is not, strictly speaking, proper, therefore, to speak of ideas "inside" my mind and objects "outside," any more than it is proper to say that my backyard, which I look at through the window in my bedroom, is "inside" my room... The viewer is inside the room, the viewed is outside the room, but the view itself is... where? In the window? In my eye? Outside? I do not know.
My point is simply that there is more work to be done here on what an idea is... I, unfortunately, cannot advance an alternate thesis. I will, however, pose the question to Descarte and Hume and Locke, all of whom seem quite confident that they know what ideas are.
Calling it a "copy" of the outside world seems right, but it makes me intensely curious, along with the stranger from Elea, what the nature of a copy is... a thing and not a thing?
