 CPI licenses a different rudimentary argument. If a person believes that one particular thing caused another, e.g., the mixture of samples of blue and yellow pigment caused the quality green to begin to exist; the inductive principle underwrites the inference that any compound similar to the first must have a similar effect. To be sure, it is often very difficult to collect evidence that two particular things have similar natures, because natures do not appear in awareness and it is impossible to examine all the effects of two individuals in an effort to learn whether the unknown natures are the same. Yet Shepherd explains that, under favorable conditions, we may come to believe they are the same with a high degree of probability (see  § 5.1).  The inductive reasoning just described is contrasted with Hume’s explanation of causal inference in terms of the habitual association of ideas and their liveliness. It is also compared with views like that of Reid, followed by Brown, on which if we once perceive that a thing of one kind follows a thing of another kind, we immediately from the conclusive belief that all things of the first kind are followed by those of the second. As Shepherd puts it, we give undoubting assent to this law on the evidence of nothing but the law itself; yet it has reference to actual things which everyone agrees can only be known from experience. This is to believe without a reason to believe, according to Shepherd (ERCE: 136–45; EASP: 144–5). To her mind, a belief is irrational if not held for a reason. 