There are three principal means of acquiring knowledge available to us: observation of nature, reflection, and experimentation.
Observation collects facts; reflection combines them; experimentation verifies the result of that combination. Our observation of nature must be diligent, our reflection profound, and our experiments exact. We rarely see these three means combined; and for this reason, creative geniuses are not common.
ATTRIBUTION: Denis Diderot (1713-1784), French philosopher. repr. In Selected Writings, ed. Lester G. Crocker (1966). On the Interpretation of Nature, no. 15 (1753).