"Our usual method" is an expression that Aristotle uses in a passage of the Politics (I.1, 1252a17-20) to describe the technique of subdividing a composite object of study into the elements that make it up, in order to study the possible combinations. What theory is the basis for this method? Is it really a usual method for Aristotle? Enrico Rini responds to these questions, starting with the analysis of some passages of the Aristotelian corpus that refer to concepts of the part and the whole. From these passages, Rini attempts to extrapolate a theory that Aristotle states explicitly only on occasion. The mereology that emerges from this study is not a formal theory, unlike the logical mereology present today, and therefore takes into account different types of the whole and the part, with a scope that has applications for the field of biology.