We're constantly making choices about what to think but sometimes don't realize we're choosing. One person, for example, may view a situation as threatening. Another, looking at the same situation, may be quite interested in the potential for good he sees in it. One person is continually thankful for the good he feels has already come into his life. Someone else, with even more reason to be grateful, may feel disappointed and querulous about the past. Choices like these are being made all the time.
As Christian Scientists, we become more aware that we're able to make conscious decisions about what to think. We learn we can choose the thoughts which flow from God—"angels" Mary Baker Eddy sometimes calls them in her writings.See Science and Health with Key to the Scriptures 581:4-7. We see that these thoughts don't just change our outlook on a situation; they heal the situation. They result in changing the evidence supposedly "out there."
But the discovery that is Christian Science has much more to it than this. Digging in more deeply, we begin to realize its vast dimension and power, and feel more of its full effect. We begin to see that the choice we can make is not simply in regard to what we think. The all-important choice is in regard to how we think.