Two kinds of inability (1): lack of something the will cannot supply
Thesis XVIII. That doctor and those who share his view insist, however, that the inability to do good found in everyone who lacks the grace of Christ needed for this or that good work does not cancel their obligation to do it, nor excuse them from doing their duty in that respect. To prove it they point out that there are two kinds of inability to carry out a commandment.
'One kind,' they say, 'comes from the lack of something that no amount of will, that is, no strength of willing, can make up. Such is the inability of someone who has no worldly goods to give alms, or no knowledge to teach a person perishing for want of instruction, and countless cases like these. Of this inability it is perfectly true that God does not command the impossible. The moment such an impossibility arises, either the commandment lapses or at least the person commanded is no longer bound to fulfil it. In impossibilities of this kind the will is also counted as the deed, since the failure to act is not the will's fault but sheer lack of power.'
Latin
XVIII. Caeterum Doctor ille, & qui cum eo sentiunt, pertendunt eam bene agendi impotentiam, quae reperitur in omnibus illis, qui destituti sunt gratia Christi ad haec vel illa bona opera necessaria, non tollere obligationem qua tenentur illa facere, nec eos excusare, quominus hac in parte officio suo fungantur. Atque ut hoc ostendant, notant duplicem esse praecepti faciendi impotentiam. Una est, inquiunt, ex defectu alicujus, quod non potest quantumlibet magna voluntate, hoc est quantumlibet fortiter volendo, suppleri. Talis est impotentia illius qui caret rebus temporalibus ad largiendam eleemosynam, & scientia ad eum docendum qui defectu instructionis perit, & hujusmodi infinita. De tali impotentia verissimum est, quod Deus non jubet impossibilia. Nam hoc ipso quo talis oritur impossibilitas, vel praeceptum extinguitur, vel certe ad illud implendum, is cui praecipitur, non amplius obligatus est. In hujusmodi etiam impossibilibus voluntas pro facto reputatur. Non est enim culpa voluntatis quod non fiat, sed pura inopia potestatis.