Agnosticism IS the most dishonest position

[Belacqua]

They define the concept "tree" and "rock" not each and every tree and rock.  Again, the role of a concept in cognition is to condense a huge number of units into one.  A unit is one of a group of similar things.  similar things vary only in their specific measurements.  The concept tree subsumes all trees that exist now, have existed, or will ever exist. We don't define each individual tree just as we don't define every unit of the concept "man".  We don't have a definition of Sally or Rob or John because these are not abstractions.  concretes have descriptions, abstractions have definitions.  If we defined every concrete we come upon that would defeat the purpose of definitions, which is the final step in concept formation.  Think of a concept as a file folder, a word as the label on the file folder and a definition as a shorthand description of what's in the folder, enough to let you know what's in there and to differentiate the contents of one folder from another.  Unit economy is the name of the game in cognition.

Right. We form concepts based on our experience. We have direct experience of a number of different things, and abstract these into an abstract concept in the mind. 

As the man said in the 13th century: there is nothing in the mind that wasn't first in the senses. (I'm not completely sure this is true, but we can go with it for now.) And as the same guy was careful to point out, the kind of thing we can sense depends on the kind of bodies we have, and the kind of animals we are. We obviously can't sense some things that other animals can sense, and there's no way we can even comment on what aliens might sense. 

So concepts are abstractions derived from concrete examples. 

And since we can't sense many things, it's almost certain that there are things in the universe we can't conceive of. The apophatic theologians are at pains to remind us that, in their opinion, some things about God are this way. Can't be sensed, and can't be conceived of. Therefore, in their view, although natural theology demonstrates the existence of God, there is still much about him that can't be conceived of by people. But the fact that we can't conceive of it doesn't mean it's not there. 

In addition, human beings have the ability to form new concepts by recombining elements of previous concepts. You can conceive of an imaginary animal, for example, though you've never seen it, by combining portions of different animals. I have experienced things that were unique, or at least seemed so to me, so I have a concept of uniqueness, and now I can imagine things that don't really exist that are unique -- that is, not members of existing groups.

We can also conceive of things that are probably impossible according to the laws of nature. For example, faster than light travel. I've never gone faster than light, but I can imagine going really fast. It appears that at least two popular movie franchises depend on our ability to conceive of this impossible thing. 

I have a concept of the Christian God in my mind. It has a clear definition: the Ground of Being. The fact that I have this concept in no way proves that it is a real thing. Nonetheless, it is a concept invented by people by combining known things in the imagination, and has a clear definition.