I thought that the "it is that which you see before you" at the end was a great closure ... and very appropriate for Jeff F, because that is what he always comes back to.
Co-incidentally, the second half of the sentence you quoted, "begin to reason about it and you at once fall into error," corresponds to the WWW quote in the post above.
"That which is before you is it. Begin to reason about it and you will at once fall into error."That is how I have it written down. Which is different from combining the two phrases above (mine and yours) into one sentence.
For what it's worth, my edition runs thusly, "It is that which you see before you---begin to reason about it and you at once fall into error."