Action as a fundamental ontological category - Im Anfang war die Tat - is the basis for action as a fundamental epistemological category, e.g. Marx's idea of "praxis". Whitehead's process ontology gives it the same ontological and epistemological status.
"The conception of knowledge as passive contemplation is too inadequate to meet the facts. Nature is ever originating its own development, and the sense of action is the direct knowledge of the percipient event as having its very being in the formation of its natural relations. Knowledge issues from this reciprocal insistence between this event and the rest of nature, namely relations are perceived in the making and because of the making. For this reason perception is always at the utmost point of creation. We cannot put ourselves back to the Crusades and know their events while they were happening. We essentially perceive our relations with nature because they are in the making. The sense of action is that essential factor in natural knowledge which exhibits it as a self-knowledge enjoyed by an element of nature respecting its active relations with the whole of nature in its various aspects. Natural knowledge is merely the other side of action. The forward moving time exhibits this characteristic of experience, that it is essentially action. This passage of nature - or, in other words, its creative advance - is its fundamental characteristic; the traditional concept is an attempt to catch nature without its passage." (Whitehead, An Enquiry Concerning the Principles of Natural Knowledge, p. 14)
In the Preface to Process and Reality he describes this ontology as "a transformation of some main doctrines of Absolute Idealism onto a realistic basis." (Corrected Ed. p. xiii)
Marx conceives of his own project in pretty much the same way. It is a correction, in the light of "idealism", of the failure of "all hitherto existing materialism" to allow for a coherent conception of a "subject" and hence for human being as "sensuous human activity, practice".
"The chief defect of all hitherto existing materialism (that of Feuerbach included) is that the thing, reality, sensuousness, is conceived only in the form of the object or of contemplation, but not as sensuous human activity, practice, not subjectively. Hence, in contradistinction to materialism, the active side was developed abstractly by idealism - which, of course, does not know real, sensuous activity as such."
Ted