When does doubt arise?
Let's suppose we ask a question. I'm drinking a cup of water. The question is, how do we know it's water? You can come to absolute, unfettered certainty that it is water. Why? Because the definition of water is, "a clear, tasteless liquid that quenches thirst." You look at the cup of water: it is liquid, it is clear, it has no taste, and when I drink it, my thirst is quenched. By definition, this must be water. There is no question: it is certain. If it turns out one of these parts of the definition is false, it must therefore not be water.
Let's suppose we ask a different question. The question is, are my perceptions reality? You cannot help but have doubt. Is this water clear, or do I merely think it's clear? I don't know. There's no way to be certain. We can be deceived. There is doubt.
What's the difference between the two questions? The first question has nothing to do with the self. It is a simple, objective question. Is this water? Of course it is. The fact of it being water proceeds from the simple knowledge of experience. I experience it to be water, and because the thing is defined by the experience (i.e. the experience of it being clear, the experience of it being liquid, the experience of quenched thirst, etc.) it must by definition be water. But do I perceive it to be water, or not? The question doesn't even make sense. It either is water, or it isn't. Perceptions are irrelevant. Do I merely think it's water? "Thinking" is the problem.
If you experience it as water, it is water. If you don't experience it as water, it isn't water. If you perceive that it looks like water and it may or may not be water, you're being pretentious.