“For illustrative purposes, let us sketch a plausible (though simplistic) scenario for the evolution of a complex category. Consider a child in the process of learning the various senses (conventional usages) of the word tree, and suppose the term is first applied, in his early experience, to such ordinary specimens as oaks, maples, and elms. Their perceptual prominence and obvious gross similarities enable the child to extract a conception that embodies their commonality, while excluding the many properties that vary from one instance to the next. Presumably this conception emphasizes intrinsic, characteristic, and cognitively salient properties (e.g. shape, size, color, brachiation, leaves); subtler and more contingent properties are either ignored or factored out. Hence the notion is fairly concrete and specific compared to many the child deals with, but as an abstraction from varied experience, it nevertheless constitutes a low-level schema. With continued usage, this initial conception becomes more deeply entrenched, and comes to be invoked for the categorization of more divergent experience. As a starting point for the gradual evolution of a complex category, it can be recognized as the category prototype.
“Suppose, then, that our child has mastered the concept [TREE] (the eventual category prototype), as well as the symbolic relationship [[TREE]/[tree]]. When he encounters a tall plant with branches, leaves, and bark he readily sees it as conforming to the specifications of [TREE] and takes it as a straight-forward instance of the tree category. When happens, now, when he first encounters a pine, which is [TREE]-like in most respects but has needles instead of leaves? He will quickly learn to call it a tree, either from hearing someone refer to it in this way or because this is the most nearly applicable term at his disposal. This usage implies the symbolic unit [[PINE]/[tree]], derived by extension from the original [[TREE/[tree]]. The two symbolic unites are identical at the phonological pole, but at the semantic pole [TREE] is only partially schematic for [PINE], since they conflict in one of their specifications (leaves vs. needles).
“The extension is based on the categorizing judgment [[TREE] —>[PINE]]. The observation of similarity permitting this judgment takes the form of a conception (TREE’) that embodies the commonality of [TREE] and [PINE] but is sufficiently schematic in relevant respects to eliminate their conflicting specifications (in particular, it must neutralize the difference between leaves and needles). Possibility this similarity-perception endures for only a fleeting moment. It is not unlikely, though, that it eventually achieves the status of a unit with a certain amount of cognitive autonomy, i.e. the child develops a schema for the tree category that abstracts away from specific properties of the foliage. The result is then the schematic network depicted in Fig. 10.2(a),
representing the different concepts that function as the semantic pole of tree together with their interrelationships. Through extension to a broader, more variegated class of objects (including pines), the term tree acquires an abstract sense that it would not otherwise have. The meaning of tree for the child at this point is not just the schematic [TREE’}, nor is it just the prototype [TREE]; rather its meaning is given by the entire schematic network, any node of which can be accessed by the phonological unit [tree].”
—Robert Langacker (1987), Foundations of cognitive grammar: theoretical prerequisites, 373-4.