Summa Theologica

In this treatise Aquinas examines human moral activity in greater detail. 

To be voluntary, an act must proceed from a principle within the agent and involve knowledge of the end. Animals perform voluntary actions in a manner of speaking, but they do not fully understand the ends of their actions and act through sense instead of reason. The operation of the will includes two elements: the immediate act of wishing, and the action that is then put into execution. By definition, the will cannot be compelled, but people can be forced to do things against their will, through violence or fear. When someone does something out of fear, the action is voluntary in the absolute sense since the will consents to the specific action.

The fact that an action is undertaken because of concupiscent desires does not make it involuntary; the will still consents to the action. On the other hand, ignorance may have an effect on the voluntariness of certain actions.  

Aquinas considers circumstances of human acts, those conditions (accidents) which underlie them. The most crucial of such questions are why and what: To what end was this action done, and what precisely was done? These circumstances must be taken into consideration, especially by theologians, when judging human actions.