You might say to yourself, "My might and the power of my hand have gained me this wealth" (Deuteronomy 8:17).
Moses warned the Israelites that upon entering Canaan and inheriting a prosperous and fertile land "flowing with milk and honey," they should not think that their own prowess had made them wealthy. Rather, they should be aware that Israel was a Divine gift.
For that generation, the challenge was not too difficult, because as Moses had pointed out to them earlier, they had personally experienced forty years of miraculous survival in the desert wilderness, fed by the daily manna and watered by a spring which accompanied them on their journeys. With such overt manifestations of Divine wonders, they would not be likely to ascribe any future success to their own strength and cunning.
Today, however, we stand many centuries away from the Biblical times. We may think that the world operates purely by natural law; that we can completely determine our own fate and fortune, and in which success or failure are due to our shrewdness in business or how much effort we exert.
Thus, Moses' message was intended for us even more than for his generation. Surely we are required to engage in work, for the Torah itself states that God will bless the work of "your hands" (Deuteronomy 14:29), but we should not lose sight of the fact that the Divine blessing, not brains or brawn, ultimately determines our fortune. The only difference between today and Moses' time is that there, God's hand was manifest everywhere, but today it is concealed.