If you are interested, the quotes were taken from
here.
I think that Randall makes a really good point here. When we speak of immaterial things, we are referring to conceptual things, but do these things exist in and of themselves? We can speak of logic, and we know that logic exists, but is there a material thing that we call logic? Another example is the concept of a horse (or any other "thing"). When we talk of horses, we know what we are talking about because we can visualize what we are speaking of. This image in our mind is the concept by which we judge the "horseness" of all things. If the thing (we perceive) compares well to the concept, we can call that thing a horse. But does this "concept of horse" actually exist? To take this idea one more step, let us apply it to God. We all have a concept of God, but does God actually exist? How can we say "that which is immaterial exists"?
I don't want to dwell on just the "God" part of my question. I really just want to know how we all deal with conpeptualization of the immaterial and its existence. We can even take it farther than conceptualization and ask "how do we know that anything we perceive is not merely a conceptualization?".