Prudence discerns worthy ends to pursue. Justice enacts means to reach the ends while giving to others and God what is due to them. Fortitude endures the attacks from without (picture a fortified fort getting pelted by the enemy). And temperance endures the attacks from the passions within us (cowardice, treachery, etc.).
Man was made by and for God. That is our ultimate end that all other ends are oriented towards. We have come from Him and will return. Unless we don’t. We know He has called us to Himself, that we have fallen, and that through Christ we can return to His grace in this life and enjoy eternal beatitude forever in the next. But we can reject the gift.
To induce us to reject the gift we have sin and the demons. Sin is within and is fought especially with temperance as we “discipline [the] flesh” to reinstate the ordered man’s life where the intellect directs the passions and not vice versa (1 Cor. 9:27). Very few of us will encounter an intelligible manifestation of demons, but they are there. They hate us and use subtle and constant means to tempt us to reject God or despair of His mercy. And against their attacks we need fortitude in the faith, because these are attacks from beings outside of us that would like us to join them in eternal misery and damnation. Our struggle against them is an irritation to them, which is why the Saints seem to wrestle in very explicit ways with them (somewhat physically).
C.S. Lewis wrote a book called The Screwtape Letters wherein a senior demon named Screwtape is mentoring, through letters, a young demon. The younger demon “loses” the man, meaning the man dies and goes to heaven. This is a very irritating thing to Screwtape, bemoans this loss and describes it to his protégé because he has seen it, when
There was a sudden clearing of his eyes (was there not?) as he saw you for the first time, and recognized the part you had had in him and knew that you had it no longer… As he saw you he also saw Them [the angels]. I know how it was. You reeled back dizzy and blinded, more hurt by them than he had ever been by bombs…
In that weird Screwtape Letters way the scene is beautiful. It reminds us that the demons do play a part, yet they are not all-powerful. They have already been defeated and are simply begging us to join then in defeat (which is Hell). Seeing this – as one day we will see very clearly – we grasp that the small moments add up. Each time we endure the small things, we are preparing for the biggest thing – the moment when the scales fall from our eyes and we see Him and His glory and His angels and Our Lady.
This is why we meditate on the goods of heaven. We acknowledge demons, but never regard or consider them. We merely move on from them. When they arrive we change the subject, look away, and cling to God. All our little acts of fortitude are small rehearsals for the moment when we are finally free from the need of fortitude, when we enter in and no longer endure. He who is faithful in little is faithful in much, Jesus said, so too our little acts of fortitude are pointing to the ultimate moment – facing death itself confidently – whether as a martyr or in the sickness of age – and we receive our reward.