power a disposition; an ability or capacity to yield some outcome. One tradition (which includes Locke) distinguishes active and passive powers. A knife has the active power to slice an apple, which has the passive power to be sliced by the knife. The distinction seems largely grammatical, however. Powers act in concert: the power of a grain of salt to dissolve in water and the water’s power to dissolve the salt are reciprocal and their manifestations mutual. Powers or dispositions are sometimes thought to be relational properties of objects, properties possessed only in virtue of objects standing in appropriate relations to other objects. However, if we distinguish, as we must, between a power and its manifestation, and if we allow that an object could possess a power that it never manifested (a grain of salt remains soluble even if it never dissolves), it would seem that an object could possess a power even if appropriate reciprocal partners for its manifestation were altogether non-existent. This appears to have been Locke’s view (An Essay concerning Human Understanding, 1690) of ‘secondary qualities’ (colors, sounds, and the like), which he regarded as powers of objects to produce certain sorts of sensory experience in observers. Philosophers who take powers seriously disagree over whether powers are intrinsic, ‘built into’ properties (this view, defended by C. B. Martin, seems to have been Locke’s), or whether the connection between properties and the powers they bestow is contingent, dependent perhaps upon contingent laws of nature (a position endorsed by Armstrong). Is the solubility of salt a characteristic built into the salt, or is it a ‘second-order’ property possessed by the salt in virtue of (i) the salt’s possession of some ‘firstorder’ property and (ii) the laws of nature? Reductive analyses of powers, though influential, have not fared well. Suppose a grain of salt is soluble in water. Does this mean that if the salt were placed in water, it would dissolve? No. Imagine that were the salt placed in water, a technician would intervene, imposing an electromagnetic field, thereby preventing the salt from dissolving. Attempts to exclude ‘blocking’ conditions – by appending ‘other things equal’ clauses perhaps – face charges of circularity: in nailing down what other things must be equal we find ourselves appealing to powers. Powers evidently are fundamental features of our world. See also DISPOSITION , QUALITIES , RELA- TION , SUPERVENIENC. J.F.H.