Jamieson, Fausset and Brown on James 2:20

20. wilt thou know--"Vain" men are not willing to know, since they have no wish to "do" the will of God. James beseeches such a one to lay aside his perverse unwillingness to know what is palpable to all who are willing to do.

vain--who deceivest thyself with a delusive hope, resting on an unreal faith.

without works--The Greek, implies separate from the works [Alford] which ought to flow from it if it were real.

is dead--Some of the best manuscripts read, "is idle," that is, unavailing to effect what you hope, namely, to save you.