Agnosticism IS the most dishonest position

[Belacqua]

When we imagine, we are selectively rearranging things we've previously perceived into a new combination that does not exist in reality.  Even if I were to imagine something that exists, like a Pear, the product of that process is not an abstraction but the mental equivalent of a concrete.  I'm unable to imagine an abstract pear.  I've got to give my imaginary pear specific measurements.  It has a specific size, shape, color.  If there's no measurement ommission, then there's no abstraction.  If there's no abstraction then there's no definition.  Instead there is a description.  My imaginary pear is 6 inches long, has yellowish green skin with a red blush on one side. It tastes sweet and slightly tart and it has a grainy texture.  I am unable to imagine a Pear with no specific color, weight, size, etc.

Something else occurred to me while I was walking around on the mountain just now.

Philosophers have generally distinguished between two different forms of imagination. 

All these guys realized that when we look at an object there is not just a straightforward mirroring of that object in our minds. They suggested that we have a mental faculty which takes in and assembles the various sense impressions to make a coherent mental image. Since the product is an image, this faculty was called imagination. Different writers used various terms. Often it's called imagio. 

The second type of imagination is the one we've been calling imagination on this thread. It is the type which assembles remembered sense-impressions into new combinations. This was often called phantasia. 

Obviously this all took a step forward when Galileo and Newton introduced their new metaphysical views. As you know, they asserted that all the qualities we perceive in our mental images are not present in the object itself. So if we take your example of the pear, they say it is not green. It reflects light of a certain wavelength, which somehow the mind represents to itself as green. But the color is "inside" the mind, not "outside" with the pear. Likewise the flavor. The pear itself has no flavor, only chemicals. The flavor is a product of the mind. 

So Kant, following these scientists, gave an even greater role to the image-making function. It not only assembles and preserves the appearance, it actually creates the qualities which are not present in the world. He called this Einbildungskraft, which is just translated as "imagination." (Literally it's more like on-forming-power.)

The key here is that what you're calling the "mental image of a concrete" is in fact a kind of abstraction. It is a creation of the mind. Even if you're looking right at the pear, what you're getting in your mind is not the noumenal pear, not the pear-in-itself, but an interpretation created by the image-making faculty. If Galileo and Newton are correct, it means that when you imagine (in phantasia) a pear-like fruit that you have never seen in reality, what you are rearranging are things which were created by your imagination in the first place. 

So when you say you're unable to imagine an abstract pear, I think this may not be correct. Because the image we have of any pear is in its way an abstraction. Can the original sense-image, assembled from the wavelengths of light and the chemicals, be given a definition? I think it can. The definition is that it's a pear. Can a mental image of a pear, recalled from memory, be given a definition? I think it can -- again, it's a pear. Can a mental image assembled from various memories to construct a pear-like imaginary fruit be given a definition? I don't see why not. Call it a "peareach," because it looks like a pear but tastes like a peach. I define it in this way.

So I think we can give definitions to imaginary abstract things.