Flow is defined as a "holistic sensation that people feel when they act with total involvement" (Csikszentmihalyi, 1975, pg. 36). If flow seems under-defined and seem intuitive, that is because the definition is based on native categories, or something that individuals identify themselves. To this end, flow is highly subjective and related to an experiential state. The essence of experience however, is that it can never be fully captured, which makes the issue of measuring flow highly problematic. Hegel (1977) aptly describes experience as being what we experience in the moment. In a particular moment, the properties of things can be said to be experienced by the senses, we experience the qualities of objects through this sensuous interaction with them. In contrast, the conception of ‘the uniqueness of the thing’ is an idea that develops independently of what is experienced of the thing. From this, one can assume that a thought is then the non-sensual component of experience.
To illustrate further, when a child first sees a red ball that she has never seen before, she experiences its properties. She immediately apprehends it; she apprehends the color red, the roundness of the ball, the smoothness of its texture, and so forth. She then develops a concept of the object as ball; a concept that is independent of all the properties previously encountered through the senses.
There are of course many interpretations of experience, but Hegel's phenomenological perspective sheds light on how a concept or definition of flow, or even play, is only part of the picture. I'm beginning to really like the notion of play as inherently contradictory (or dualistic if we must) since it is experienced in the moment-to-moment but only made sense of after the fact.