It is also possible in principle to build a stainless-steel ladder to the moon, and to write out, in alphabetical order, all intelligible English conversations consisting of less than a thousand words.
While I agree with the general point that it's important to consider impossibilities in fact, I'm not quite sure I agree where he's drawing the line between fact and principle. Does the compressive strength of stainless steel, and the implied limit on the height of a ladder constructed of it, not count as a restriction in principle?
It just takes some imagination. Hollow out both the Earth and the Moon to reduce their gravitational pull; support the ladder with carbon nanotube filaments; stave off collapse by pushing it around with high-efficiency ion impulse engines; etc.
I agree, though, that philosophers often make too much of the distinction between "logically impossible" and "physically impossible." There's probably no in principle possible way to hollow out the Earth significantly while retaining its structure; etc.
Another month has passed and here is a new rationality quotes thread. The usual rules are: