Agere sequitur esse is a principle of Thomist ontology (St Thomas Aquinas - the study of being) according to which the action of each entity depends on the designated nature of the entity itself.
That means, for instance, that a cup functions as a drinking vessel directly according to its design. If it were without a receptacle it wouldn’t completely fulfil this function – and without a receptacle, it wouldn’t truly be a cup. It might be something else, but it certainly wouldn’t do what a cup was intended to do.
Let’s apply this principle to men. A person functions as a man because of his anatomical, chemical and neurological structures. Without the Y chromosome, testosterone and different-sized parts of the brain called the amygdala and hippocampus, a human wouldn’t operate fully as a man, with a man’s generative properties, strength and specific neurological capabilities. Without these, he wouldn’t be a man. He might be something else, but he certainly couldn't do what a man was created to do.
It follows, then, that every object in existence must act in the particular way it was made, and for the task it was made to do.
Additionally, the object is, in itself, an act. Being active derives from being in existence. Anything in existence has agency, the capacity to act according to its existence or to manifest its capacity through its existence. Here we differentiate active motion from active purpose. A table may appear inert, but it remains in a continuous state of readiness to act as a surface for other objects. That is its active purpose.
As sentient creatures, we realise that agere sequitur esse is a moral principle: we are obliged to do what we are created to be. The moral ‘ought’ is founded on the ‘is’ and one’s moral duties are grounded in one’s being. This is the given reality of the individual. At a basic level, a cup ought to hold liquid; if it doesn’t there is mild moral outrage and the offending article is disposed of. At a human level, a man ought to do what a man has been created to do; if he doesn’t, there can (and should) be extensive moral outrage.
For Aquinas, this having to-be descends from being brought into existence. A cup is created by us out of the necessity to hold liquid and must therefore carry out that function. It cannot sweep a room because it was not brought into existence to do so. It could be used to hold pens or even be used as a weapon but in each case there is some level of aberration in its nature. Not only that, the cup must be the best receptacle for drinking from, otherwise it will be superseded by something else. Thus an entity tends towards its own perfection.
But Man, since he is a creature of God , does not limit himself to the ontological characteristics of a simple object like a cup, but holds to moral behaviours (the moral ‘ought’) consonant with his divine creation. Therefore man must aspire to divine perfection (in a way that a cup cannot).
This should not be understood superficially in the sense that the action is sequential to being. That is, "I’ve been created a man and could choose, if I like, to adopt manly attributes". This is a trap of modern thinking on gender, which says a boy might have been born male, but that doesn’t mean he will necessarily continue being one. If he chooses to wear a pink dress, for example, then he might turn out to be female!
It should also not be understood in the sense that the action simply corresponds to the nature of being. This is often the modern accusation of baseness thrown at men: men do just what men do, and we may or may not be able to change it whether we like it or not.
No, for Aquinas, fulfilled being derives its meaning from the act of existing according to our design. Take a dance, for example. A dance is brought into existence by the act of dancing and stops when the act stops. The more energetic the dancer, the more exuberant the dance. When the dancer stops dancing, the dance is gone. Likewise, fulfilment as a man comes from our existence coinciding with the actions required for being manly. Manliness starts and ends with masculine deeds. There should be no distinction between who you are and what you do. People should be able to say, I know he’s a man! Why? Because he does the manly things expected of men!
In the Garden of Eden, Adam was given four manly tasks: dominion over creation, to be fruitful and multiply, to till the land and to guard creation – primacy, procreation, provision and protection. Keep these tasks before you, pray for clarity of mind and strength of will to carry them out wisely, and hold on to the truth that we have been created for a purpose and in that purpose we find our fulfilment.