I am convinced that it is impossible to expound the methods of induction in a sound manner, without resting them upon the theory of probability. Perfect knowledge alone can give certainty, and in nature perfect knowledge would be infinite knowledge, which is clearly beyond our capacities. We have, therefore, to content ourselves with partial knowledge-knowledge mingled with ignorance, producing doubt.

The Principles of Science: A Treatise on Logic and Scientific Method (ed. 1913)