Benedict Spinoza's way of describing reality was to say it has two aspects (or attributes in older translations) which he called Extension and Thought. Today this dualism seems antiquated and rooted in medieval philosophy, but to my mind there is more to it than that. Let’s take his idea further, which perhaps we may Attempt without too much violence to his overall philosophy.
Extension is easily translated to physical reality – anything with extension in space. Thought is more difficult because Spinoza saw it as the thoughts of God, but his God was extremely remote, a kind of eternal logical definition of what the universe is. Let’s translate Though as universal logic, or simply as logic. Nobody has to agree with this by the way, it’s merely a bit of casual exploration.
Aspect 1 – physical reality.
Aspect 2 – logical reality.
We can describe anything we like in terms of Aspect 1 or Aspect 2, but must never try to mix them. Aspect 1 and Aspect 2 describe exactly the same fundamental reality, but they are incommensurate. So we may describe a person in terms of flesh, blood, biochemistry and neuroscience (Aspect 1), or we may define a person logically in terms of behaviour, belief or personality (Aspect 2). In Spinoza’s terms, humans are what he called “modes” of Aspect 1 or Aspect 2, modes being temporary configurations.
Now Spinoza also said that there are an infinite number of other Aspects to reality, but these are entirely beyond our comprehension. Let’s lump them together and call them Aspect 3. In a sense, Aspects are similar to dimensions, which would make us two-dimensional beings in a universe of infinite dimensions. So understanding a person in terms of Aspect 1 and Aspect 2 as described above is an incomplete understanding. Aspect 3 is missing.
It follows that humans cannot have a complete understanding of anything. Nothing. Everything we try to analyse has Aspect 3 missing from the analysis. Aspect 3 is the incomprehensible shadow behind our reality.