 A second guiding but unintuitive commitment of Spinoza’s epistemology has come to be known as the “attribute barrier”. A Spinozistic “attribute” is a descendant of the Cartesian “principal attribute” (AT 8a.25): roughly, it is the most basic qualitative kind under which something can fall. For example, to say that something is a mental thing or a physical thing – or, in Spinoza’s and Descartes’s terminology, “thinking” or “extended” – is to understand it in terms of its particular “attribute”.