From Abraham Kaplan's book The Conduct of Inquiry (1964).
An observation in science is first of all something done, an act performed by the scientist; only thereby is it something seen, a product of the process in which the scientist is engaged. As process, observation is a part of what Nagel calls "controlled investigation". Scientific observation is deliberate search, carried out with care and forethought, as contrasted with the casual and largely passive perceptions of everyday life. It is this deliberateness and control of the process of observation that is distinctive of science, not merely the use of special instruments (important as they are) – save as this use is itself indicative of forethought and care. Tycho Brahe was one of the greatest of astronomical observers though he had no telescope; Darwin also relied heavily on the naked eye; De Tocqueville was a superb observer without any of the data-gathering devices of contemporary social research.Observation is purposive behavior, directed towards ends that lie beyond the act of observation itself: the aim is to secure materials that will play a part in other phases of inquiry, like the formation and validation of hypotheses. When observation is thought as passive exposure to perception, its instrumentality is left out of account. The scientist becomes a voyeur, finding satisfaction in the unproductive experience of just looking at nature. No doubt there is always some gratification in uncovering secrets, exposing what is hidden; but the scientific motivation is more mature in its demands. In science, observation is a search for what is hidden, not just because it is hidden, but because its exposure will facilitate an intimate, sustained, and productive relationship with the world.