 As Suárez approaches these issues, the question of what a cause is differs crucially from the question of how a cause actively brings about its effects—two questions which were often run together in later periods of philosophy. According to Suárez, one might truly say, for instance, that a doctor is a cause of health, without yet understanding the precise activity of the doctor in virtue of which he is a cause, how it is that his activity on a specific occasion qualifies as an instance of healing—how, that is, the cause exercises its causality (DM XII 2.13). To have a full understanding of the science of medicine, then, one must understand the causes of health and, further, how those causes exercise their causality, that is, how those causes manage to have the effects they have. So too, then, with the causes of being. To understand being, we must understand what brings being about, in the broadest possible sense—what is responsible for being (ens). Thereafter, we must come to terms with what causes individual beings (entia) to come into existence, or to change their manner of existence once they have come into existence. This involves reflection on all the causes of being, however many and of whatever kinds they may be. It further involves reflection on the narrower question of how the causes of being exercise their causality—how precisely, that is, such causes come to be responsible for being. This, then, licenses, or indeed requires, Suárez to engage in a thorough examination of causation itself.