ACT is rooted in the pragmatic philosophy of functional contextualism, a specific variety of contextualism that has as its goal the prediction and influence of events, with precision, scope and depth. Contextualism views psychological events as ongoing actions of the whole organism interacting in and with historically and situationally defined contexts. These actions are whole events that can only be broken up for pragmatic purposes, not ontologically.
Because goals specify how to apply the pragmatic truth criterion of contextualism, functional contextualism differs from other varieties of contextualism that have other goals. ACT thus shares common philosophical roots with constructivism, narrative psychology, dramaturgy, social constructionism, feminist psychology, Marxist psychology, and other contextualistic approaches, but its unique goals leads to different qualities and different empirical results than these more descriptive forms of contextualism, seeking as they do a personal appreciation of the complexity of the whole rather than prediction and influence per se.
ACT itself reflects its philosophical roots in several ways. ACT emphasizes workability as a truth criterion, and chosen values as the necessary precursor to the assessment of workability because values specify the criteria for the application of workability. Its causal analyses are limited to events that are directly manipulable, and thus it has a consciously contextualistic focus. From such a perspective, thoughts and feelings do not cause other actions, except as regulated by context.
Therefore, it is possible to go beyond attempting to change thoughts or feelings so as to change overt behavior, to changing the context that causally links these psychological domains.
Further information on functional contextualism is available here