First capacity: soundness of means and instruments
This is the possession of sound faculties and adequate means. It concerns general eligibility to perform a deed and the applicability of obligation.
Second capacity: the particular power of the act
This is the capacity in its specific connection to the deed occurring, and it relates to the time of actual performance.
Someone who affirms the first capacity before action need not affirm that the second exists beforehand. Equally, locating the second alongside the deed does not deny the earlier availability of the first.
This classification resolves much of the apparent difficulty concerning the timing of obligation and performance.