See and Avoid
Actually a very good question. There was some research done by the then DERA CHS at Farnborough which showed that, in a typical fast jet, the human eye/brain is not sufficient to scan a large enough area in time to take the necessary action to avoid collisions.
A lot of pilots will, like Tourist above, just come out with the mantra "see and avoid", but this is pretty pointless if you don't see the threat!
The study resulted in the widespread fitting of High Intensity Strobe lights to military aircraft, as well as some other interesting recommendations which were not taken up, like light ac and helicopters flying the opposite way through flow arrows - the idea being that they then have the threat in front of them (and are therefore more likely to see it) rather than being hit by a jet from behind - howwever the equation is a delicate balance because while you have twice (or three times) as many pairs of eyes looking, you also have less time because the velocities sum rather than subtract.