The paper argues against the very commonly held view that whenever a substance may be said to be the cause of something, a fuller and metaphysically more accurate understanding of the situation can always be obtained by looking to the properties in virtue of which that substance was able to bring about the effect in question. Paul Humphreys’ argument that when a substance is said to have produced an effect, it always turns out to be an aspect or property of that substance which brought about the effect in question is examined and criticized; it is argued that it is based on an illegitimate application of Mill’s Methods of Difference and Agreement to the case. Mill’s methods, it is suggested, are methods of empirical, not of ontological enquiry. The paper then turns to examine an argument by Mele which appears to depend on a structurally rather similar assumption that if there is nothing about a subject which could explain why she does one thing rather than another, it cannot really be up to that subject which thing occurs. It is suggested that, too, the inference is faulty, and that once it is rejected, one common objection to libertarianism—the argument from luck—might be more readily met.