William Burkitt on James 1:8

By a double-minded man, we are to understand one that is divided in his own thoughts between two different ways and opinions, as if he had two minds, or two souls. Many such there were in the apostles' days, judaizing brethren, that sometimes would sort with the Jews, sometimes with the Christians. Many such there are in our days, divided betwixt God and the world, between holiness and sin; like a needle between two load stones, always wavering to and again, pointing frequently to both, but never fixed to either. Such a man, says our apostle, is unstable in all his ways; that is, in all his actions.

Learn hence, that whilst men's minds are divided between God and their lusts, they must necessarily lead very anxious, uncertain, and unstable lives, always fluctuating in great anxiety and uncertainty; for he is always at odds with himself, and in perpetual variance with his own reason. Where men's minds are double, their ways must necessarily be unstable.