I am attempting to unify three principles that are often, or usually discussed in separate contexts, and to emphasize that they are three aspects of the same computational principle. Abstraction is generally discussed in the context of symbolic representations of concrete objects. Compression is discussed in the context of computer data compression. Invariance is discussed in the context of a recognition system being tolerant to certain transformations. I am saying that these concepts are all related in perception, i.e. that evidence of either abstraction, compression, or invariance is suggestive of a higher-level abstracted representation relative to a lower-level reified one. This elaboration on the issue is now included in the new draft.