Originally Posted by
Poseidon
I forgot which books but several mention them being psychers. The great houses of the navigators have a higher propensity for being psychic and is the only way for them to rise in rank. They don't practice the violent methods but more meditation to see into and through the warp. It's why in several ship books they are the first to be attacked when the gellar field goes offline. The strongest psychic resonance gets attacked first being the best soul for daemons. Also they have the third eye which sees psychically through the warp.
Also link [url]http://warhammer40k.wikia.com/wiki/Psyker[/url]
Although several times I've read of navigators having a small pychic attack.