In Chapter 4 of my Truth, Probability, and Paradox (hereafter TPP) I tried to give an account of dispositional properties or powers. However, I failed to make my views clear to some readers, partly because I developed my own position only gradually and by way of criticism of others, and some who have understood it have found my key arguments unconvincing. Consequently, though my views have not substantially changed, it may be worth while to present my positive conclusions more bluntly and so, I hope, more clearly, and to reinforce the arguments that support my chief claims. At the same time I shall be able to relate what I have to say about dispositions both to the account of causation which I have since offered in The Cement of the Universe (hereafter 'CU') and to views about primary qualities and real essence developed in Problems from Locke (hereafter 'PL'). This account will not replace the more discursive treatment in TPP, but it may clarify and strengthen it. Some mere complications may first be got out of the way. Singlymanifested dispositions like fragility can be distinguished from multiplymanifested ones like acidity. Again, a sure-fire disposition, one which the thing simply will manifest whenever the appropriate stimulus and/or conditions are present can be distinguished from a probabilistic or even merely possibilistic one, where even when the conditions are appropriate there is only some chance, or perhaps a mere possibility, that the thing will then manifest the disposition. Many ordinary dispositional terms are indeterminate between these alternatives: is something explosive only if in certain circumstances it will explode, or is it enough that it may explode or has some chance of exploding? Similarly, if a man is to count as irascible, how inevitable must it be that he will get angry if provoked? Again there is a distinction between active and passive dispositions or powers: fragility is a passive power, a thing's disposition to break when something is done to it, and sugar has the passive power of being soluble in water while water has the active power of dissolving sugar. But this distinction is pretty arbitrary; we could cover the same facts by saying that