Inasmuch as we consider the principles of mobile being as constituting the object in its becoming, we have three principles, two of which are opposed to one another.
To become something else, even in an accidental way, is to assume some new form and lose the previous one. The privation is the form-from-which and the form, simply, is the form-to-which. The subject (matter) is named so because it is the subject of these forms in its changing and static state.
When we consider the principles of an object as it is in act, meaning in actual existence, then we are left with two principles, form and matter. This is the hylemorphic conception of material substances. Form and matter are not opposed to one another since opposition cannot, of itself, produce unity, and the object, as one object, is necessarily unified. Therefore its principles, considered as constituting the actual object, cannot possibly be opposed to one another or contrary to one another.