 The isomorphism of sensory sensations as a whole and their respective external causes as a whole enables us to have a sort of inferential knowledge about sensible objects in addition to their existence. The order and comparison of sensations provides information about the external world in all those respects in which there is structural similarity between the two domains. Although there are in principle no limits to the abstract similarities that might be discovered by students of nature, Shepherd stresses similarities that are sufficiently close to being complete and exact that we need do little more than attend to relations exhibited by accumulated masses of sensations in order to comprehend them. This is the advantage of our natural tendency to associate objects of perception and the sensations under which we are bound to think of them. It obscures the metaphysically important distinction between inner and outer existence, but it facilitates descriptive knowledge of the external world. Shepherd illustrates this by an analogy with algebra. Just as algebraic signs stand for unknown quantities which can be “measured, valued, and reasoned upon by their signs”, so sensations are signs for their causes which exist under relations similar to those we observe, calculate, or demonstrate in the domain of sensations. As mentioned, we typically give sensations and their causes the same name. So we may speak, think, and reason about external objects as if they really existed under the forms in which they appear (EASP: 27–8, 47–9, 161–6). Shepherd finds it important to observe that no part of this arrangement is subject to skepticism: the relation between sensations and what they signify is assured by CP and CPI; relations among sensations are assured by observation and reasoning, and their interpretation is assured by the proportionality that, according to CPI, must hold among causes, considered as a whole, and the sensations they effect, considered as a whole. 