 There are at least two possible answers the acquaintance theorist might give. A view we might call inferential externalism suggests that to acquire inferential knowledge that Q by inferring Q from P it is enough that P makes highly probable Q. The relation of making probable, itself, can be interpreted in many different ways. A detailed discussion of interpretations of probability would take us too far afield, so let us be content with a few very brief remarks (see the entry on  interpretations of probability).  One might try to understand probability in terms of frequency. Roughly the idea is that P makes Q probable when P and Q form a pair of propositions of a certain kind, where usually when the first member of a pair of propositions of that sort is true then so is the second. Alternatively, some philosophers would follow Keynes (1921) and argue that there are relations of making probable that hold between propositions analogous (in some ways) to the relation of entailment that holds between propositions. On this view, when P makes probable Q, it is a necessary truth that P makes probable Q (though that necessary truth is perfectly consistent with the fact that the conjunction of P and X might not make probable Q).