  The first is a largely empirical argument based on the two rational functions of the understanding. The understanding discovers the abstract relations of ideas by demonstration (a process of comparing ideas and finding congruencies and incongruencies); and it also discovers the causal (and other probabilistic) relations of objects that are revealed in experience. Demonstrative reasoning is never the cause of any action by itself: it deals in ideas rather than realities, and we only find it useful in action when we have some purpose in view and intend to use its discoveries to inform our inferences about (and so enable us to manipulate) causes and effects. Probable or cause-and-effect reasoning does play a role in deciding what to do, but we see that it only functions as an auxiliary, and not on its own. When we anticipate pain or pleasure from some source, we feel aversion or propensity to that object and “are carry’d to avoid or embrace what will give us” the pain or pleasure (T 2.3.3.3). Our aversion or propensity makes us seek the causes of the expected source of pain or pleasure, and we use causal reasoning to discover what they are. Once we do, our impulse naturally extends itself to those causes, and we act to avoid or embrace them. Plainly the impulse to act does not arise from the reasoning but is only directed by it. “’Tis from the prospect of pain or pleasure that the aversion or propensity arises...” (ibid.). Probable reasoning is merely the discovering of causal connections, and knowledge that A causes B never concerns us if we are indifferent to A and to B. Thus, neither demonstrative nor probable reasoning alone causes action.