 The reasons for this supposition are two: (1) we cannot make sense of the concept of an unsupported quality or of ideas subsisting by themselves, (2) we know from experience that “a certain number of these simple Ideas go constantly together” or “exist together” (Essay II.XXIII.2–3). Although Locke thinks of these reasons as totally compelling, he sees quite well that they do not justify any claim as to what a substance or a thing really is, what its nature or constitution consists in. Thus he never gets tired of emphasizing that we only have a confused idea of substance (a claim also made by Leibniz about three-quarters of our knowledge, although he held that we have a clear concept of what substance is), and repeats quite often (at least three times in Essay book II, chap. XXIII alone) that