May 24 2007
It is an action of the intellect:
_On the contrary,_ Our Lord said (John 17:3): "This is eternal life: that they may know Thee, the only true God." Now eternal life is the last end, as stated above (A. 2, ad 1). Therefore man's happiness consists in the knowledge of God, which is an act of the intellect.
Thomas goes on to develop this considerably.