Many, if not most, of our highly prized ‘laws’ of physics cannot be adequately rendered as statements of regular association among the values of ‘categorical’ quantities, I have argued.63 This is true even if we do not balk at the concept of natural necessity and are willing to add that the associations hold ‘by law’. They are rather ascriptions of capacities. They tell us what capacities a system will have by virtue of having a given property. The law of gravity is one example. A system of mass M has the capacity of strength GMm/r2 to move another object of mass m a distance r away towards itself. I call this the gravitational capacity. My second thesis is a commonly shared one. Ascriptions of capacities do not reduce to conditionals involving only categorical properties. I shall here discuss two questions about these theses: 1) Why think of capacities as akin to dispositions or powers; and 2) Why allow them in science? Before tackling the first question, I shall first try to figure out what features we expect to be characteristic of dispositions and powers themselves
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