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.


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?

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Posted by Keith E. Buhler


  1. I think that that is a good point. Ideas are not completely seperate from what they are of. Otherwise, you end up being a solipist. Plus, and this is just an intuition right now, I think it makes your own ideas inaccessible to you. I.e. if you are thinking about an idea you kind of become external to it, consequently any ideas you have about the idea are also completely seperate from that idea.

    However, the ‘ofness’ of ideas is also tricky. Doesn’t it seem to merely push the problem up a level instead of solving it?


  2. Eric, yes,

    the “ofness” of ideas is exactly the puzzle.

    How can, say, the copy of the tree on the cover of “Seeds of Deception,” the book to the right of this comment, be a picture OF a real tree…? It seems that it must be the tree and not be the tree, at the same time. In what respect?

    I want to say “it appears like a tree, though it does not have some of the same properties, like being made of wood, being 3-dimensional, being 18 feet tall and so son.” But this is insufficient. If the stranger were here, he would ask me “what do you mean by ‘appears’?” And I would say “It looks like it has the same properties of wood, 3-dimensionality, height, etc.” But it does not have the same properties, so how can it look like it does? That’s exactly the question.

    “Being like a thing” is tantamount to saying, “being and not-being that thing.”

    Perhaps Plato gives us a clue, also from the Sophist. The stranger distinguishes between two forms of image-making… one (eikastics) is the replication of the proportions of an object, such as height and width, etc., as well as color. The other form (phantastics) replicates the thing as it seems to us, in perspective. The example used is a very large statue of a person wherein the head is actually made disproportionately large so as to appear correct to viewers down below. This is true of Michelangelo’s David, for instance.

    I’m not sure what this gives us, but it does help me say, of the picture of the tree, that it is an exact replication of the proportions of a tree.

    Does this help us with the question about mental copies? Are Descartes’ and Hume’s ideas of similar proportion to the actual objects?


  3. I’m working on a more indepth reply, but your post also made me think of an interesting correlations with Christian artwork:

    1. Byzantine = eikastic (no perspective, realistic/spiritual proportions)
    2. Renaissance = phantastic (perspective, unrealistic/unspiritual proportions)

    Now, an interesting way this all comes together is with some icons I saw in the Orthodox Church. The icons were weird in that much of the representation of the figures was of form 2, except for their faces and some other articles which were of form 1. This had a weird effect on me, almost of something transcending the piece of art as an external object and connecting with me.


  4. Think how trippy a piece of art that combines 1 & 2 seemlessly would be:)


  5. Err, actually on second thought I think the analogy between my comments and the video might be kinda obscure, so don’t expect a whole lot. The main thing is that it produced a similiar effect in me as seeing the Orthodox art (although the video is admittedly a bit disconcerting).


  6. Ok, now I’ll try for a reply with a bit more substance.

    Intuitively to me, for there to be an identity relationship between the object and your idea, there must be an identical thing that they share. In order for your idea to correspond to the object’s reality, the thing that they share must be at least part of what makes the object real.

    Yeah, yeah, forms again. I’ll try and stay away from the Torrey line, or do so critically. Please blow the above thoughts apart. Show me why the Greek answer isn’t compelling.


  7. Well Eric, all you have done is again push the problem one level higher, for how do you distinguish between the form and the object? The thing in you mind must be both the form and not the form for you to make that distinction, silly boy.

    In that case how about this:

    The thing in my mind is the memory of my experience of particular object X. I both remember the impressions that were made on me and remember that these impressions did not come from me, which comprise what I would call my idea of X.


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