English term (edited): golden mountain

This is a philosophical rather than a linguistic question.

The "golden mountain" comes from David Hume's A Treatise on Human Nature (1739-40), vol. I, part 1. Hume's argument here can be summarised as follows:

An experience is stronger and more vivid than the memory of that experience.
Lively and forceful perceptions of the mind can be called impressions.
Thoughts or ideas are less forceful.
The imagination seems to be unrestrained, but actually all complex ideas are derived from simple impressions, by compounding (putting together), transposing, augmenting or diminishing mental materials derived from experience (senses or feelings).

For example, the idea of a golden mountain can be traced back to our experience of "gold" and "mountain": it compounds or joins these two simple ideas, with which we are acquainted.

Hume is not referring to a mountain that looks golden (perhaps because of a trick of the light), but a mountain that is actually made of gold. This is something which, necessarily, we have not experienced. We can think about it, but that thought is simply a conjunction of the ideas of gold and mountain derived from our experience.

Following this, he gives a second example: a virtuous horse. Again, we cannot have experienced this; it is an idea that combines or compounds the simple ideas of virtue and horses, both of which we know separately.

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Note added at 1 hr (2011-07-21 22:10:17 GMT)
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The very interesting passage Stephanie quotes from a letter to the London Magazine illustrates the lively debate that took place on Hume's ideas in the following decades. The writer of this letter is actually saying that the idea of a golden mountain cannot be contained in the mind; the best a person can do is recollect a mountain and the colour of gold, but this is not the same thing: "We cannot properly be said to imagine what does not, or has not, really existed."