For Kant, the thing-in-itself is linked to the unknowable noumenon, an object or entity that exists beyond sensation and perception. Nevertheless, Maritain thinks that in our tangible aesthetic subsistence, we deal with essences embodied in concrete reality. This corresponds to Aristotle’s metaphysics as the science of ‘being qua being.’ His categories of being not only describe the way we think about what is, but also describe the way things are. Thus, the categories of being are both mental constructs of thought and, most significantly, descriptive of realty. In this way thinking gives access to actuality, and you never find forms by themselves, as separate entities. You only find form composite with matter, as the primary kind of being is found in hylomorphic particulars.
“By form I mean the essence or very nature of the thing.” – Aristotle