  Yet our discoveries about co-existences of properties—though they are merely contingent particulars, or, insofar as they are applied beyond the particular cases we have actually observed, are mere probability—can yet qualify as real knowledge. To do so, our ideas must meet certain conditions. The complex idea that we refer to a substance must comprise all and only those simple ideas that we have found to co-exist in nature. Here Locke is concerned to show that sensitive knowledge deserves its appellation, since it can be distinguished from arbitrary or otherwise poorly grounded claims (e.g., that fluidity has been found to co-exist with brittleness, in a single substance and at a single given  temperature.[14]) Sensitive knowledge is far less than scientia, but far more than ungrounded opinion.