A nose is a nose is a nose. Yeah, right—if we've gotten anything from Tristram Shandy, it's that a nose is anything but a nose.
Tristram swears up and down that "nose" doesn't mean "penis," saying that he's depending "upon the cleanliness of my readers' imaginations" and insisting that he is taking the "clean" rather than the "dirty" sense of nose (3.31.4-5). But let's get real. When Tristram's great-grandmother is able to rewrite the marriage contracts because her soon-to-be husband has "little or no nose" (3.31.2), it's pretty clear that she means he has a tiny penis.
Or is it? You can ask a pretty big question here—not just, "What does this particular literary symbol mean," but "What do literary symbols mean in general?" That is, how do symbols work? How do we know when something is a symbol and when it's just a prop?
One reason noses (or penises) matter is that, for Mr. Shandy, they're the source of all potency, not just sexual but political, economic, and marital. The constant threat to noses and penises suggest that they're linked, and that masculinity is constantly being undermined.
Tristram finds a novel (ha!) way of dealing with the threat to his manhood. Remember, both his nose and penis have been injured. In place of the penis, Tristram uses another symbol of power: the pen.