  In modern algebra it is well-known that one cannot, in general, apply
ordinary equational reasoning when dealing with partial algebras. However Boole
did not know this, and he took the opposite to be a fundamental truth, which he
called the Principles of Symbolical Reasoning in his 1854 book {\em Laws of
Thought}. Although Boole made no mention of it, his Principles were clearly a
generalization of the earlier publications on algebra by the Cambridge
mathematician Peacock. After a detailed examination of Boole's presentation of
his Principles, we give a correct version that is applicable to his algebra of
logic for classes.
