The second half (after the semi-colon) is a restatement of the first
half, but in slightly less general terms. So we need something
that essentially repeats the same thought contained in the first half.
The comparison is between our reaction to thought (the mind, things
mental) and our reaction to sensation (perceptions). The claim in
the first half is that our reaction is often as strong to thought as to
sensation. The idea of "reaction" is modified to "have an
influence" in the second half. Also "as strong as" is modified to
"the sensations can even be less strong (weaker)(in influence) than
thought (things mental). In spite of the modifications, as I said
earlier, the two halves of the statement are really saying the same
basic thing.
I take the meaning of the first half to be an allusion to the fact that
we often take action based on our mental conception of reality rather
than based on what our physical perceptions may be telling us.
(For some reason optical illusions came to mind.) So among the
choices, the one that struck me was the one that, when substituted in
the blanks, said that very ordinary sensations may be telling us one
thing, but we will be more influenced by the symbolic power of mental
constructs when we act. (The hysteria surrounding the burning of
the U.S. flag comes to mind. People's sensations tell them that
it is just a piece of colored cloth in a pattern very
familiar to them, but their mental constructs (cultural baggage, if you will) influences them to see it as a
symbol
of the entire country and its history. Since the conceptual
(mental) is stronger than the purely perceptual (sensational), people
get very upset about flag burning. I believe this may be the kind of "moral" reaction the author is talking about.)
I hope that helped.
CJ