 The expression on the left-hand side of ‘\(\eqdf\)’ (i.e., \(\ldots X\ldots)\) is the definiendum of the definition, and the expression on the right-hand side is its definiens—it being assumed that the definiendum and the definiens belong to the same logical category. Note the distinction between defined term and definiendum: the defined term in the present example is \(X\); the definiendum is the unspecified expression on the left-hand side of ‘\(\eqdf\)’, which may or may not be identical to \(X\). (Some authors call the defined term ‘the definiendum’; some others use the expression confusedly, sometimes to refer to the defined term and sometimes to the definiendum proper.) Not all definitions found in the logical and philosophical literature fit under scheme (2). Partial definitions, for example, fall outside the scheme; another example is provided by definitions of logical constants in terms of introduction and elimination rules governing them. Nonetheless, definitions that conform to (2) are the most important, and they will be our primary concern.