As a type of characteristically vital action we may take _nutrition_, in which occurs a phenomenon which Galen calls _active motion_ ([Greek: drastikê kinêsis]) or, more technically, _alteration_ ([Greek: alloiôsis]). This active type of motion cannot be adequately stated in terms of the passive movements (groupings and re-groupings) of its constituent parts according to certain empirical "laws." Alteration involves _self-movement_, a self-determination of the organism or organic part. Galen does not attempt to explain this fundamental characteristic of _alteration_ any further; he contents himself with referring his opponents to Aristotle's work on the "Complete Alteration of Substance" (p. 9). The most important characteristic of the Physis or Nature is its [Greek: technê]--its artistic creativeness. In other words, the living organism is a creative artist. This feature may be observed typically in its primary functions of _growth_ and _nutrition_; these are dependent on the characteristic _faculties_ or powers, by virtue of which each part draws to itself what is proper or appropriate to it ([Greek: to oikeion]) and rejects what is foreign ([Greek: to allotrion]), thereafter appropriating or assimilating the attracted material; this assimilation is an example of the _alteration_ (or qualitative change) already alluded to; thus the food eaten is "altered" into the various tissues of the body, each of these having been provided by "Nature" with its own specific faculties of attraction and repulsion. [Sidenote: The Three Categories.] Any of the operations of the living part may be looked on in three ways, either (_a_) as a [Greek: dynamis], faculty, potentiality; (_b_) as an [Greek: energeia], which is this [Greek: dynamis] in operation; or (_c_) as an [Greek: ergon], the product or effect of the [Greek: energeia].[3] [Sidenote: Galen's Method.]