Suppose a goldsmith takes his tongs and puts some gold into the furnace to melt it. If he blows on the heat too much, it will get too hot, but if he sprinkles too much water, it will cool down. If he constantly takes it out and looks at it, it will not reach refinement. But if he does all these things from time to time, aware of the nature of gold, it will become easily molded and bright.Anguttara Nikaya
In the same way, there are three qualities that a practitioner should pay attention to--concentration, determination, and equanimity. If he pays the right attention to these at the right time, then his mind will become like gold, pliant and brilliant and pure.