Two contraries cannot co-exist inside the same subject. Aristotle wrote that and it seems to say the obvious; something can't be light and dark at the same time.
However, in terms of what's happening inside our souls it seems that contraries can indeed co-exist inside the same subject.
At any given moment, inside us, we are a mixture of light and darkness, sincerity and hypocrisy, selflessness and selfishness, virtue and vice, grace and sin, saint and sinner. As Henri Nouwen used to say: We want to be great saints, but we also don't want to miss out on all the sensations that sinners experience. Our lives aren't simple.
We live with both light and darkness inside us and for long periods of time, it seems, contraries do co-exist inside us. Our souls are a battleground where selflessness and selfishness, virtue and sin, vie for dominance.
But eventually one or the other will begin to dominate and work at weeding out the other. That's why John of the Cross picks up this philosophical axiom and uses it to teach a key lesson about coming to purity of heart and purity of intention in our lives. Because contraries cannot co-exist inside us, there's something vital we need to do. What?
We need to pray regularly. Contraries cannot co-exist in us so if we sustain genuine prayer in our lives, eventually sincerity will weed out insincerity, selflessness will weed out selfishness and grace will weed out sin. If we sustain genuine prayer, we will never, long-term, fall into moral rationalization.
If we sustain genuine prayer in our lives, we will never grow so blind to our own sin that we will begin to have morally exempt areas in our lives.
Being faithful to prayer will ensure that we will never, long-range, live double lives because what prayer brings into our lives, a genuine presence of God, will not peacefully co-exist with selfishness, sin, rationalization, self-delusion and hypocrisy.
At some point in our lives, we will either stop praying or stop our bad behaviour. We won't be able to live with both. Our biggest danger then is to stop praying. This advice is eminently practical: We cannot always control how we feel about things. We cannot always control how we will be tempted.
And none of us has the strength to never fall into sin. Our incapacity to fully actualize ourselves morally leaves us always short of full sanctity.
But there is something that we can control, something beyond the wild horses of emotion and temptation. We are beset by many things, but we can wilfully, deliberately, with discipline and resolve, show up regularly to pray. We can make private prayer a regular discipline in our lives. We can commit ourselves to the habit of private prayer.
If we do that, irrespective of the fact that we will have to work through long periods of dryness and boredom, eventually what that prayer brings into our lives will weed out our bad habits, rationalization and sins. Two contraries cannot co-exist inside the same subject. Eventually we will either stop praying or we will give up our sin and rationalization. Nobody can be praying genuinely on a regular basis and be blind to his or her own sinfulness.
Our task then is to sustain private prayer as a habit in our lives, even if we have neither the insight nor the courage to see and address all the double-standards and moral blind spots in our lives. What comes into our lives through prayer, often more imperceptible than visible, will eventually weed out both our sin and our rationalizations about it.
This is akin to what Ronald Knox once taught about the Eucharist. For him, the Eucharist is the singular, vital, sustaining ritual within Christian life.
Why? Because Knox believed that, as Christians, we have never really lived up to what Christ asked of us. We have never really loved our enemies, turned the other cheek, blessed those who cursed us, lived fully just lives or forgiven those who hurt us. But we have been, he submits, faithful to Christ in one major way: We have been faithful in celebrating the Eucharist, to that one command.
Just before he left us, Jesus gave us the Eucharist and asked us to continue celebrating it until he returned. For 2,000 years, awaiting that return, we've been faithful in doing that, no matter how unfaithful we have been in other ways. We have continued to celebrate the Eucharist and, in the end, more than anything else, that has been the one thing that has called us back, again and again, to fidelity.
The habit of private prayer will do the same thing for us. Since two contraries cannot co-exist inside the same subject, eventually either we will stop praying or we will stop sinning and rationalizing. The greatest moral danger in our lives is that we stop praying.