What Dewey is talking about here seems to be a dispositional account of spontaneity. What is spontaneous for us is what we have prepared ourselves to be spontaneously receptive to or productive of. Preparation takes the form of practicing until we have habituated ourselves to a certain kind of response in certain kinds of situations. What we are to practice at, however, remains a little unclear. Dewey begins another description of this notion of spontaneity with a reference to the miracle of mind.
Emotion is the moving and cementing force.
It selects what is congruous and dyes what is selected with its color, thereby giving qualitative unity to materials externally disparate and dissimilar. It thus provides unity in and through the varied parts of experience. The miracle of mind that Dewey is referring to here is a reference to the amazing work that gets done by emotion in the construction of our experience. Experience now seems to be constructed, and certainly the reference to qualitative unity refers to the construction of meaning, but the role of agency remains unclear. Dewey says that perception itself will be creative.