It's an interesting position and had me rethinking things a bit, but the way I look at it, the actions themselves are negative; it's their boundary conditions which are different. Take for instance embark/disembark. In pseudo-mathematical terms, I would tend to think they increment or decrement one's embarkedness, with an upper boundary of 1 (aboard), and a lower boundary of 0 (ashore). The non-existence of values >1 (super-aboard) or <0 (anti-aboard) shouldn't affect the relative polarity of the actions themselves. I think. Looking through the rest of the list, there's a variety of different boundary conditions. Prove/disprove would range from 1 to -1 (1=proven, 0=asserted but untested, -1=proven false), entangle/disentangle seems to range from 0 to infinity (because you can always be a little more entangled, can't you?), and please/displease is perhaps wholly unbounded (if we imagine that humanity has an infinite capacity for both suffering and joy).