The confusion that results is one of ignorance. If what is purely sensible is hard to understand, so is that which is purely intelligible. That is to say, some things are below our understanding and others are above it. Take the mammal again. We cannot imagine a mammal as such, but we can easily imagine a particular mammal, such as a dog or a cat.
Concept is a produce of the intelligence, but image is a product of the sense.
As Maritain warns, concrete examples actually may mislead us if they indulge the imagination rather than the intelligence. Metaphysics is difficult precisely because it is the most abstract and least concrete of sciences. It is natural for us to think along the sensible level using concepts. Thus, we may confuse intelligence with imagination, and say we don t understand something when in fact we cannot imagine it. That brings us back to substance .