Although this topic can be discussed at great length, one nice explanation was recently discovered and written about by Bhayahari dasa, a Washington, D.C. area devotee. He reiterates sastric evidence that compares the agriculturist, the rains and time. In order for grains to manifest and become harvestable and edible, the farmer has to sow seeds. Then there has to be rain, and lastly time must elapse in order for the grain to manifest. The planting can be compared to free will, and the rain to destiny. Time can be looked at as a third factor without which neither free will or destiny can manifest results.