Hayes (2005) will tell you that "people suffer". Suffering is a universal human condition. Yet he also acknowledges the courage and determination of human beings, pointing out that we often persevere and succeed even in the most difficult conditions. The ACT philosophy is therefore that humans can live meaningful, productive lives even though they will inevitably experience pain, sadness, and other undesirable emotions, memories, and experiences. So a basic motivation of human beings is to find meaning and live a productive life.

The philosophy behind ACT is known as functional contextualism. Functional contextualism is distinguished from other approaches by three underlying values: (a) It is holistic, (b) the role of context is central in understanding events, and (c) the criterion for truth is pragmatic. Loosely translated, these three principles mean that the person can't be separated from her parts or her surroundings (both physical and psychological, present and past). Also, the values of explanations for events is judged by the extent to which they work for the person (pragmatism). There is a behavioral flair to this approach, in which antecedents and consequences of actions are evaluated, but always in context, with an eye toward the function of the behavior.

ACT is based on the theoretical foundation of relational frame theory (RFT), developed by Hayes. With roots in radical behaviorism, RFT starts with the premise that language and cognition are learned, along with a set of relationships between phenomena, called relational frames. An example is the comparative and evaluative frame, which links things together with terms such as better than, bigger than, and prettier than. Although they are first learned in a specific context, these relations can be established and generalized even without direct experience; that is, humans can learn by observing or through language. We can also transfer this learning to other situations or, in ACT language, contexts. We begin to learn these relationships as babies, and never stop learning such associations; we accumulate relations.

Behavior research demonstrates that behavior, including linguistic relations, is never really unlearned. So the sometimes strange relations we learn stick with us, and worse, messing with them in the way that traditional cognitive approaches do may only elaborate them and make them stronger. Also, trying not to think of these thoughts is not effective; it may work for a while, but then the avoided thoughts come back. According to Hayes, these principles apply to behavioral dispositions and emotions, too.

ACT therapists take a health rather than a disease model of psychological dysfunction. The first lesson of ACT is that healthy people are those who accept their humanness, with all of its good and bad, and work toward a life informed by personal values. Because we are all going to suffer and also be happy, we need to simply go with the flow.

Health in ACT terms is cognitive flexibility; we don't let our current thoughts and feelings get in the way of working toward our valued long-term life goals. All of the other processes specified in ACT theory contribute to cognitive flexibility, which is why it is in the centre of the figure. The basic ACT processes can be classified into mindfulness and acceptance processes and commitment and change processes, which presents another way to look at psychological health from this perspective.

In terms of the ACT processes, healthy means that we should be open to our experience, particularly our feelings, but it doesn't rule our world. We mindfully accept our experience, but avoid our natural tendency to regard our thoughts and associated feelings as truth -- a process called cognitive fusion. Healthy people know that thoughts are just thoughts, and no more than that. They are likely to be reviewed for their workability, not as truths. Healthy thinking is defused.

In contrast, folks who are experiencing psychological dysfunction are likely to be fused with their thoughts; they are immersed in them, take them as Truth, and believe that they always cause their behavior. Where people go wrong is when they try to respond to, control, or avoid parts of their inner lives -- specifically, those thoughts or feelings that we find aversive. Devoting ourselves to obsessively managing our experience limits our options to act in ways consistent with our values. Experiential avoidance is at the root of many of our human woes. According to ACT theory, this doesn't work because these relations can't be unlearned.

Another feature of dysfunction in the ACT model is the conceptualized self, seen when the individual is stuck on one version of the self and can't unhook across the various life contexts she experiences. Sometimes this conceptualized self is helpful, such as when it helps us describe ourselves to others. At other times they are not helpful, such as when we receive contradictory information, which can cause unwelcome emotions and spark experiential avoidance. Also, rigidly holding negative conceptualizations is associated with relational frames that are invested with truth and associated with negative emotions that the individual wants to banish.

Assessment can take many forms in ACT. On a very basic level, the counselor tries to understand the client and see where she might be stuck (conceptualized self) or avoiding experience.

ACT theorists acknowledge that formal assessment tools can be helpful, but caution that standard inventories most often register the presence of the symptoms, not their function (how they are related to the client's language system). Hayes and colleagues (2012) recommended a "Work-Love-Play" assessment, in which the therapist attempts to understand the client's functioning across valued aspects of life. There are also specific ACT measures such as the Acceptance and Action Questionnaire.

Although many sources bypass the issue, the core conditions of warmth and genuineness are important in ACT. Counselor and client are equals in ACT, and the belief that the client can change is emphasized. Part of being equal is that the ACT therapist is willing to self-disclose when doing so facilitates the client's progress.

The ACT therapist is also a witness, holding the client accountable for choosing and following through with valued action. However, both must be careful to view this process as a living one; it is a journey that will surely take different paths than anticipated, and the outcomes must be viewed flexibly. Getting stuck on particular ways of getting there or on particular goals can be seen as a form of fusion, so the process is one of trial and error, guided by the values identified by the client. In ACT language, clients are expected to "commit and slip".

The person of the therapist is important in ACT. Counselors must apply the principles of ACT to themselves. For example, the ACT therapist must be aware that her own thoughts are simply verbal events entwined in context. Accepting her own experience makes it more likely that the therapist can accept the client's. Therapists must not get stuck on what "should be" in therapy, avoid certain paths of exploration or experience, or choose actions that are not consistent with their values as therapists.

The primary goal of ACT is to help clients behave in ways that are consistent with their values, even when uncomfortable, unwanted private experience tells them to do otherwise. It is not enough to think about doing this; ACT requires committed action. A necessary prerequisite to committed action is psychological flexibility.

Another way to look at ACT goals is through the concept of willingness, defined as "being open to one's whole experience while also actively and intentionally choosing to move in a valued life direction. From a narrower perspective, the ACT therapist tries to help clients change the functions of experiences (mainly thoughts), not their form. When clients seek help, it is because they are frozen; their inner experience is dictating their behavior rigidly. The ACT therapist wants to change the social and verbal meanings of these relations. After all, if a thought is not "true", it doesn't need to drive our behavior.