Argument from Partial Understanding

The argument from ignorance is a fallacy which runs like this:

1. We have not proven beyond all doubt that X is true.
2. Therefore, X is probably not true.

Its close cousin is the argument from incredulity:

1. I would hardly be able to believe it if X were true.
2. Therefore X is not true.

It also takes this form, which I call the egoistic form of the argument from ignorance:

1. If X were true, then I would know it were true (or I would understand claim X).
2. I don't know for a fact that X is true (or I don't understand claim X).
3. Therefore X is not true.

I just saw a version of this argument applied to the question of scientific realism, call it an anti-realist argument from the position of egoistic partial understanding:

1. Realists believe in the literal truth of scientific statements about both observable and non-observable (or theoretical) entities, while anti-realists grant this level of belief only for statements about observable entities.
2. Bridges are observable, concrete is observable, and cracks in concrete are observable; molecules and molecular bonds are not so observable.
3. I'm an engineer, and I understand bridges, concrete, and stress fractures; I never really understood what was going on in my organic chemistry class.
4. Therefore engineers like me should be anti-realists.

What the student in question thought was really mind-blowing about this argument was that until he came up with it, he thought he was a realist. But then he realized that was only a case of naive realism, and that engineers like himself can treat most scientific claims as if they're true, whether or not they're really true, and that this is good enough for their purposes.

The real absurdity is that bad reasoning or not, there's validity to this explanation of what engineers' belief in science requires!