One of Scotus's arguments (from the Questiones subtilissime Scoti in metaphysicam Aristotelis), as very roughly translated by me from some random version of the Latin I came across once (and thus the translation is to be taken with a grain of salt both on the textual side and on the translation side). The argument is actually quite simple in structure, since it is a parity argument, although one has to read the last few sentences closely. I find this is commonly true of Scotus; he is very difficult to read and interpret, but the logical structure of the arguments is generally very simple and elegant once you manage to win through to it.
Anything active regards as its primary object such a passive thing, not this passive thing. For example, what in general is able to heat regards as its primary object what in general is heatable, not this or that particular one. Likewise from the other side, the passive, what is heatable (and this either in general or as any particular heatable), regards as its primary object what is able to heat, not this or that particular one, but in general.
From this it follows that whatever is contained under the primary object of something is object, per se, of the same. Thus what is able to heat regards as its object, per se, what is heatable, and (from the other side) the heatable, whatever is able to heat. But it is possible for something to be active with respect to A in the same way that something else is active with respect to A, and for something to be passive with respect to A just as something else is passive with respect to A. Therefore it, as active, has itself, as passive, for an object just as it does anything else; therefore it is able to act on itself just as on another.
