The first concrete sign of a conclusion is the phrase "After all" at the beginning of the third sentence. This phrase is a support indicator that tells us what comes after the phrase is designed to provide support to the claim immediately preceding the phrase. So, the author wants to support the conclusion that the brain's mental representation of the external world necessarily leaves out a lot of detail. Why should we believe this conclusion? Because (1) the world is much larger and more complex than the human brain, and (2) an object cannot completely represent stuff that is larger and more complex than that object.

You don't need to understand the logic of the argument in order to correctly identify this conclusion, but it's still something you should learn to improve your overall LSAT skills. You can think of the second premise as a rule that the author applies to the human brain. Since an object can't completely represent stuff that's bigger and more complex than itself, a brain (which is an object) can't completely represent stuff that's bigger and more complex than itself.

The other premise gives us a fact that interacts with this rule. Since the external world is larger and more complex than the brain, this persuasively leads to the conclusion that a brain can't completely represent the external world. In other words, the brain must be leaving something out of its representation of the world.

Based on the analysis above, we're looking for a restatement of the claim that the brain's mental representation of the external world necessarily leaves out much detail.