Constancy here refers to being consistent and firm in the manner of the stoics rather than foolishly rigid. Montaigne quotes Socrates answering Laches, who said fortitude was “standing firm in rank against the enemy,” by asking, “What? Then would it be cowardice to beat them by giving ground?”
I admire much about the Stoic sensibility, a significantly more complex philosophy and approach to life than most attribute to it. To be a good stoic is not to be unbending or unyielding, but to not let that bending and yielding penetrate into the core of who we are and what we think, to not let those feelings “penetrate right to the seat of reason, infecting and corrupting it.”
Perhaps there is some idealized stoic who has achieved their equivalent of enlightenment who feels nothing the way the rest of us do and so does not have an obligation to react as we should, but the day-to-day reality is that we must learn to absorb what comes with the right amount of sway and movement to allow for it without accommodating it and giving it a home. We must learn to be emotionally earthquake proof in the way the buildings that can withstand the movement of the earth are not the strongest and most solid, but the ones with some flex.