(1) Evidentiary grounds of belief are (_a_) memory, which is plainly indispensable if we are to learn by experience; and (_b_) testimony, which must be trusted if language is not to be useless and social co-operation impossible: both these grounds are supposed to rest upon the primary rock of previous perception, but are slippery and treacherous. Memory is only valid so far as it truthfully represents original experience, and testimony only so far as it presents (i) a valid memory, (ii) correctly reported. Hence in serious matters precautions must be taken against their fallibility: otherwise they are not good evidence. A specious memory, so far as it is false, is imagination; and false testimony, so far as it reports (i) a false memory or (ii) an invention of the reporter, is also imagination. Testimony gathers force, as a cause of belief, with the numbers and consideration of those who support it, and is especially strengthened by their unanimity; but, as a ground of belief, it depends only on their knowledge and truthfulness. A third ground of belief is (_c_) inference; which is necessary to all original adjustment of our conduct to the future or to unperceived circumstances, but highly fallible, and constituting the chief problem for the exercise of Logic when that science arises: especially to explain the conditions of valid observations and experiments, of probability, of the conclusion of an argument being covered by its premises, and of the sufficiency of verification. False inferences that cannot be verified are imaginations.