Non-self applies to both, but mental phenomena reveal it more directly through their impermanent, dependent nature.
The Buddhist teaching of anatta (non-self) applies universally to all phenomena included in the five aggregates (skandhas): form, sensation, perception, mental formations, and consciousness. The Buddha taught that clinging to any of these as a permanent, independent self is a fundamental delusion that perpetuates suffering. However, the doctrine operates somewhat differently across these categories because physical form and mental phenomena have distinct characteristics that make their selflessness apparent through different mechanisms.
The five aggregates framework itself demonstrates this universal application. In the Anattalakkhana Sutta (SN 22.59), the Buddha systematically applies the three marks of existence—impermanence, suffering, and non-self—to each aggregate without exception. He asks monks whether form, sensation, perception, mental formations, and consciousness should be regarded as "self, or as belonging to self, or as in self, or as having self in it." The consistent answer is no. This textual foundation establishes that anatta is not selectively applied but comprehensively addresses all dimensions of experience.
Physical form appears to have solidity and persistence that seems contradictory to non-self teaching, yet this very apparent stability reveals its selflessness upon investigation. The body constantly changes at microscopic and macroscopic levels—cells die and are replaced, the body ages, and nothing in material form remains constant from moment to moment. Despite these changes, we habitually identify with the body as "I" or "mine," creating a sense of continuous self that obscures the aggregate's actual nature.
The Pali Canon frequently uses physical form to illustrate non-self precisely because the identification is so strong and therefore so instructive to examine. In the Samyutta Nikaya passages on the five aggregates, form is presented first and treated with particular attention. The form we call "the body" is dependent on causes and conditions—food, elements, earth, water, fire, and air. It arises when conditions are present and ceases when conditions are absent. This conditional arising (pratityasamutpada) directly contradicts the notion of an independent, self-governing self. The apparent solidity of the body makes it an excellent teaching vehicle for demonstrating how even the most seemingly substantial phenomena lack independent essence.
Mental phenomena reveal non-self more immediately because their impermanent, dependent, and reactive nature is transparent to direct experience. Consciousness, thoughts, emotions, and perceptions arise and pass away with obvious rapidity when carefully observed. Unlike the body, which we experience as a relatively stable object, mental states demonstrate their momentary quality through direct introspection. Each thought depends on conditions: sense contact, attention, mental factors, and prior mental states. No thought is self-originating or self-sustaining.
This difference in transparency is why meditation practice often focuses on observing mental phenomena. In vipassana (insight) meditation, practitioners observe how feelings, perceptions, and thoughts continuously arise and disappear without any permanent controller behind them. The Visuddhimagga (Path of Purification), compiled by Buddhagosa in the fifth century, describes this investigation in detail. When investigating the mental aggregates, practitioners directly experience that awareness itself is not a unified, permanent witness but a series of momentary consciousness-moments (citta-ksana in Sanskrit; citta-kkhanika in Pali), each arising and ceasing in dependence on conditions. This immediate experiential access makes the non-self nature of mind more obviously apparent than in the case of form.
Both physical form and mental phenomena are fundamentally interdependent, and this mutual conditioning equally demonstrates non-self in each domain. The body influences the mind (a painful physical injury affects consciousness and emotion), and mental states influence the body (anxiety affects breathing and posture). Neither can be understood as independent or self-arising. This bidirectional causality extends to external conditions as well—environmental factors affect both body and mind, and both body and mind shape how we perceive and interact with the environment.
The doctrine of dependent origination (pratityasamutpada) encompasses this total interdependence. Form does not exist independently of the sense faculties and their objects; consciousness does not arise without contact and sense impression. The Mahanidana Sutta (DN 15) traces this intricate web of conditioning, showing how each phenomenon is embedded in a network of causation that renders independent selfhood impossible. This principle applies equally across the physical and mental domains, suggesting that while the mechanisms revealing non-self differ between form and mental phenomena, the fundamental principle operates identically.
While non-self applies equally to form and mental phenomena, the degree to which this truth is immediately obvious differs significantly. Physical form appears stable, coherent, and persistent; the body we inhabited ten years ago seems like the same body. This apparent continuity masks the fact that nothing material remains unchanged. Mental phenomena, by contrast, display their momentary nature openly. Thoughts and sensations continually flash in and out of awareness; their transience is undeniable even to casual observation.
This differential in apparent obviousness has practical implications for practice. Beginners often find it easier to recognize non-self in mental phenomena through meditation observation, yet insight into the non-self nature of the body is equally essential for complete understanding. The body's apparent solidity offers resistance that, when penetrated through investigation, yields profound wisdom. The Mahagosinga Sutta (MN 32) describes how different practitioners may access understanding through different doors, reflecting these varying degrees of accessibility depending on disposition and experience. Advanced practitioners eventually recognize that the apparent difference in stability between form and mind is itself an illusion created by attention and habit rather than reflecting any genuine difference in their fundamental nature.
The ultimate teaching of anatta requires seeing through both the apparent permanence of form and the obvious impermanence of mental states to recognize the same fundamental characteristic: the absence of independent, unchanging essence in either domain. This integrated understanding marks genuine insight (panna). When the Dhammapada states that "the mind is the forerunner of phenomena," it points to the centrality of mental understanding, yet the teaching of non-self cannot be considered complete if one merely intellectually understands the impermanence of thoughts while still clinging to the body as self.
Full awakening, as described in the Pali Canon, involves recognizing the non-self nature of all five aggregates without remainder. The arahat (accomplished one) has abandoned the conceit "I am," which operates across physical identification and mental self-sense equally. The teaching that applies equally to form and mental phenomena because both equally lack self-nature is therefore not merely a theoretical consistency but a practical requirement for liberation. The Buddha's consistent application of anatta to all aggregates, the systematic investigation in meditation practice, and the requirement for complete understanding all confirm that the doctrine applies with equal force and equal necessity across the spectrum of physical and mental phenomena.