depth first search

Observations and perceptions may be psychological, but observability is not.

The Logic of Scientific Discovery, pg. 103

Thus the real situation is quite different from the one visualized by the naïve empiricist, or a believer in inductive logic. He thinks that we begin by collecting and arranging our experiences, and so ascend the ladder of science. Or, to use the more formal mode of speech, that if we wish to build up a science, we have first to collect protocol sentences. But if I am ordered: ‘Record what you are now experiencing’ I shall hardly know how to obey this ambiguous order. Am I to report that I am writing; that I hear a bell ringing; a newsboy shouting; a loudspeaker droning; or am I to report, perhaps, that these noises irritate me? And even if the order could be obeyed: however rich a collection of statements might be assembled in this way, it could never add up to a science. A science needs points of view, and theoretical problems.

Ibid., pg. 106