By way of clarifying the definition of a mental function as “an actually or possibly observable event in behavior,” we may quote from Spearman’s presentation of the distinction between “observation” as a mental function, and “observation of birds’ nests.” Spearman says: “Suppose, for instance, that a school boy has surpassed his fellows in the observation of birds’ nests. His victory has, no doubt, depended in part on his capacity for the general form of activity known as ‘observation.’ But it has also depended on his being able to apply this form of activity to the matter of birds’ nests; had the question been of tarts in the pastry cook’s window, the laurels might well have fallen to another boy. A further influence must have been exercised by the accompanying circumstances; to spy out nests as they lie concealed in the foliage is not the same thing as to make observations concerning them in the open light of a natural history museum. Again, to discover nests at leisure is different from doing so under the severe speed limits prescribed by the risk of an interrupting gamekeeper. The boy’s rank may even depend largely on the manner of estimating merit. Marks may be given either for the gross number or for the rarity of the nests observed; and he who most infallibly notes the obvious construction of the house-sparrow may not be the best at detecting the elusive hole of the kingfisher.” One cannot, therefore, identify and measure “observation.” One can only measure “observing birds’ nests, of all kinds, at leisure,” or “observing rare birds’ nests, under stress of pursuit,” and so forth, which are “actual or possible events in behavior.” As one may glean further from Spearman’s discourse, it has been shown that most of the mental functions performed by men are not elementary, but consist of the coördination of complex factors, capable of analysis. Reading the English word “cat” from a printed page is, for instance, a very complex function.