#1 is a complicated way of saying that almost everything that a distinguished elderly scientist has an opinion about is possible. Did Clarke really believe that truly impossible things will cause elderly experts to be unsure? Or did he really just mean "almost everything is possible"?
1) When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
2) The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
3) Any sufficiently advanced technology is indistinguishable from magic.