I find in myself innumerable ideas of things which, though they may not exist outside me, can't be said to be nothing. While I have some control over my thoughts of these things, I do not make the things up: they have their own real and immutable natures. Suppose, for example, that I have a mental image of a triangle. While it may be that no figure of this sort does exist or ever has existed outside my thought, the figure has a fixed nature (essence or form), immutable and eternal, which hasn't been produced by me and isn't dependent of my mind.