I started reading Aristotle's On the Soul tonight. I'd like to show you a quote from it:

Hence a physician would define an affection of soul differently from a dialectician; the latter would define e.g. anger as the appetite for returning pain for pain, or something like that, while the former would define it as a boiling of the blood or warm substance surrounding the heart. The one assigns the material conditions, the other the form or account; for what he states is the account of the fact, though for its actual existence there must be embodiment of it in a material such as is described by the other. Thus the essence of a house is assigned in such an account as 'a shelter against destruction by wind, rain, and heat'; the physicist would describe it as 'stones, bricks, and timbers'; but there is a third possible description which would say that it was that form in that material with that purpose or end. Which, then, among these is entitled to be regarded as the genuine physicist? The one who confines himself to the material, or the one who restricts himself to the account alone? Is it not rather the one who combines both?

— Aristotle, On the Soul I.1.403a.29-403b.8 (emphasis added by me)

In his Metaphysics, Aristotle outlines four causes for each thing that exists:

1. Material cause - the matter a thing is made of (e.g. wood, nails, and glue)
2. Formal cause - the form or shape the matter takes on (e.g. that of a desk)
3. Efficient cause - the motive force or art that made the matter take on its form (e.g. the art of carpentry)
4. Final cause - the purpose for which a thing exists (e.g. to provide a surface for writing and setting things on)

In Aristotle's account, you must be able to state or explain all four of a thing's causes in order to say you understand the thing. As best I can tell, in the above quote he is talking about the material and formal causes. He says the "genuine physicist" is the one who knows both the material and the formal cause, not just one or the other. He mentions the final cause too, but that seems to be the main point.

In the modern world, we have a very highly developed method of inquiry that is very good at discovering and explaining material causes of things: the scientific method. It probably crosses over into efficient causes sometimes, and maybe even into formal causes (e.g. psychology might try to explain things in terms of explanations like that of the dialectician in the above quote), but things tend to get weird and a little shady when it does (see the replication crisis in psychology).

Certain branches of science in particular concern themselves almost completely with material causes. I'm thinking of the ones that come up when evangelical Christian apologists argue with atheists: astronomy, geology, astrophysics, evolutionary biology and the like. The methods of these sciences are very good at explaining things in material terms, and making accurate predictions based on those explanations. They are like Aristotle's "physicist" who defines a house as "stones, bricks, and timbers".

We make a huge mistake when we think we think these sciences can tell us about the other causes. Evolutionary biology can explain how a human heart works and how it changes over time, but if we expect it to tell us what man is for (final cause), we're going to be disappointed. Same for astrophysics and the purpose of the universe.

I think we make an even huger mistake when we, consciously or unconsciously, assume because sciences only investigate material causes, that material causes are all that exist. If our sciences can't investigate something it must not exist, right? So we become hyper-focused on material causes because that's what our sciences investigate. This single-cause thinking is the mistake of modernity.

It's worth emphasizing that the Aristotle quote from above is from his work On the Soul which is about, you guessed it, the nature of the soul. The word translated as "soul" is psyche (ψυχή) in the Greek, a word closely connected to the Greek words for wind and breath. In Latin it's anima (the Latin title of On the Soul is De Anima). The soul is the breath of life or the animating principle of a thing, that which allows a body to move through its own will. An animal is distinguished from inert matter by its possession of an anima.

"Soul" seems to mean something different to us now from what ψυχή meant to Aristotle, or even from what anima meant to Aristotle's Latin translators. To Aristotle it's the form (in the sense of a formal cause) of a living thing and not a material thing itself. It's not material, so biology can't investigate it, and now the soul in modern culture is mostly relegated to being either a metaphor ("the soul of our great nation") or some kind of vague thing spoken of by religious people.

We don't tend to think about our souls much, if at all. We don't consider the possibility that souls could be in a better or a worse state, and that these states could affect our material lives in a profound way. We don't pay attention to the health of our souls, trying to move them into better states. We mostly don't even have any idea of how to do this. Chalk it up to the modern problem of single-cause thinking; we're all very stuck on the material.