I agree that there is such a thing as an instantenous classification of sensory information the mechanism of which we are largely unconscious (e.g. facial recognition) but I am inclined to disagree that classification precedes sensory information. My position is that sensory information comes first and classification comes second. However, the switch between the two can occur so fast that it becomes difficult to differentiate between the two.

The worry is that if see a mental image while perceiving a tree, then how do we know there is a tree at all? It could be just like a dream tree. That naturally lends itself to skepticism, where the context is undermined by holding all of perception in doubt. — Marchesk

The purpose of reasoning is to make guesses regarding something that is unknown (i.e. something that hasn't been experienced or at the very least memorized.) Thus, the fruits of reasoning are necessarily fallible.

Does the act of perceiving a tree make us aware of the same sort of thing as it would while dreaming, hallucinating, visualizing? — Marchesk

If I understand you correctly, what you're asking is this: does the act of seeing a tree with your own eyes give you the same kind of experience that the act of, say, visualizing a tree does?

If that's what you're asking, the answer is no. There's a clear difference between the two kinds of experience. Visualization lacks the richness that seeing with your own eyes has.

But suppose that this weren't the case. Suppose that they were equally rich. Suppose that visualizing a tree was just as clear as seeing it with your own eyes. Would that make visualization non-mental? Of course not. This is because whether something is mental or not depends on context. You need to look at how your experience relates to what was in the past. It is this relation, which is never complete, because past is not finite, that determines to what degree what you're looking at is either mental or non-mental.

When we say that a phenomenon is "mental" what we mean is that it belongs to the category that we represent with the word "mind". Thus, if you want to determine whether any given phenomenon is mental or not, you have to know the rules of the category "mind". What does the word "mind" mean? In other words, what phenomena does it include and what phenomena does it exclude? That is the relevant question. Unfortunately, it is not an easy one.

Simply seeing a tree with your own eyes is not enough for the tree to be considered non-mental. What if you're inside some sort of virtual reality, for example? You need context.