Beauty is all around us. It mystifies us. It enamors us. It bewilders us. In a way, the more we try to understand beauty, the more we realise we are dealing with a philosophical giant. An entire branch of philosophy is dedicated to understanding beauty, being Aesthetics.
So, what can we learn about beauty at face value? We see that beauty is a combination of qualities that pleases the intellect. Beauty often shows us something that is inherently good such as nature, sex, virtue, or truth. In this way, beauty is a sign for us to see a priori (through speculation) or a posteriori (through experience) what is good. From a priori, we can understand the goodness of immaterial things like truth and virtue. From a posteriori, we can understand the goodness of material things like nature or sex.
However, we must not be confused that every time we experience pleasure that it is therefore by extension beauty. Sex, while pleasurable, is not always beautiful. The beauty in sex lies not in it's pleasure but in the romantic and intimate connection between two people and the potential life that can be created through sex. This is why many people who pursue sex for the sake of it's pleasure often feel hollow or empty after their experience. They see sex superficially and not for the beauty that lies in sex. This helps us explain that while beauty can help us see what is good, these good things can also be misused in ugly ways. Nature can be exploited and we end up with rivers of dirty water or seas filled with rubbish. Sex can be exploited through objectification and we end up with instability and suffering in hollow relationships, or mental illness because of anxiety around sex, or mistrust because of how people value sex, or depression through low self-worth with obtaining sex, etc.
Pleasure is, while intellectually connected to beauty, still distinct from it. Beauty is more than pleasure. Pleasure deals with what is gratifying to the senses, whereas beauty is what is gratifying to the intellect. This is why things that are seen a priori as beautiful are higher than things experienced a posteriori. This is because things considered speculatively don't provide pleasure to the senses which can deceive us with what is truly beautiful. This is how we can see that truth and virtue are beautiful and in themselves good. However, this brings up a question as to whether things like Truth are good because they are beautiful or whether they are beautiful because they are good. If Truth is beautiful because it is good then what makes Truth good? I would answer that beauty in itself does not make something good but merely presents a sign to whether something is good. The reason for this is because it would inherently create circular reasoning. Truth is good because it is beautiful and it is beautiful because it is good. Therefore, Truth is beautiful because it is good but it is not good because it is beautiful. Truth is good because it relates to what is perfect. Truth relates to what is perfect through reason (i.e. 2+3=5) and what is perfect is good because nothing is lacking in what is perfect.
In this way we can also understand beauty. Beauty relates to what is perfect. This also explains why there are varying degrees of beauty because things detract from perfection in smaller or greater degrees.
From this conclusion on beauty we can then look back at these other forms of beauty and see why they cease being beautiful through their distortions. Truth is in itself a perfection of knowledge about reality. When we detract from truth in anyway it becomes false or 'less than true' which is not as pleasing to the intellect as something which is completely true. From this we can also understand perfection as being towards the purpose of a thing or it's final cause. The purpose of Truth is the knowledge of reality. When the purpose of Truth is realised then we find beauty and perfection.
Likewise, in order to understand beauty and perfection in material things we must also have a complete understanding of their purposes or final causes. It can be argued that the purpose of sex is the unitive and procreative. Unitive in the sense that it brings two people together and strengthens their relational bond and procreative in the sense that it is for generating new life and propagating a species. Sex which isn't directed towards these two purposes will inherently be imperfect in some degree.
Nature can also be understood in a similar way. It can be argued that the purpose of nature is to give health to the world. When nature can no longer give health to a particular environment then it is said to be imperfect or ugly.
So, since beauty is about the perfection of a thing, why does it please our intellect? Neuroscience seems to see no difference in seeing something beautiful and seeing something desirable (source). I, however, would argue that beauty is a lot more than just seeing something that is desirous. Beauty, is our longing for perfection and hence why it is so closely related to religion. Beauty pleases our brains because it inspires positive emotions of hope, love, and even faith. This can also be seen through Aquinas's argument from degrees. The argument goes that objects have a certain quality to them (such as beauty), and if an object has a lesser degree then there must be another which has the highest degree of that quality. Therefore, there must be a being with the highest possible degree of all qualities and this is God.
I think from understanding this argument we can start to appreciate how beauty and God are so intertwined. If God is the highest degree of perfection and beauty, then when we experience beauty it is directing our emotions of love, hope and faith towards God. Love, because we love what pleases us. Hope, because we long for more of what we love. Faith, because we trust in what we hope for.
Therefore, if God is perfect beauty then it would be unimaginable to consider how beautiful God is. God's beauty would be infinitely more beautiful than the combination of all the beautiful things we have ever experienced. This is why if we want to understand beauty we must first understand God.