The four marks in plain language
**Self-mark (我相, ātman-saṃjñā)**: the cognitive construction "I" — the sense that there is a unified subject having experiences. When you feel a sensation and think "I feel this," the "I" part is the self-mark operating. **Other-mark (人相, pudgala-saṃjñā)**: the cognitive construction "other person" — the sense that there are distinct persons over there, separate from the "I" over here. When you see someone across a room and register them as "another person, not me," the other-mark is operating. **Sentient-being mark (眾生相, sattva-saṃjñā)**: the cognitive construction "beings" — a category of "all beings that are like me in having experience." The mark that groups "me and all other sentient beings" into one class distinguished from inanimate matter. **Lifespan-mark (壽者相, jīva-saṃjñā)**: the cognitive construction "duration of existence" — the sense that there is a continuous self persisting through time, a lifespan with a beginning and end. These four together form the cognitive skeleton of ordinary personhood: a unified subject, distinct from other subjects, grouped with them as beings, persisting through time.
Why four and not one
A simpler version would be: "a bodhisattva has no sense of self." The Diamond Sūtra could have stopped there. It doesn't — because "no sense of self" is ambiguous and can be half-dropped while the other cognitive marks remain intact. A practitioner might report no-self and still operate cognitively on other-marks (treating other people as distinct existent entities). Or might drop self and other while retaining the sentient-beings mark (distinguishing humans from stones). Or might drop self, other, and beings while retaining lifespan-mark (still experiencing their own continuity through time). The four-mark structure catches each of these partial moves. A bodhisattva in the Diamond Sūtra's technical sense has dropped all four — which is radical and, in practice, rare. The four-mark analysis also corresponds to four classical Indian philosophical problems: the self (ātman debates), the other (problem of other minds, solipsism), the being (problem of universals and categories), the lifespan (problem of personal identity through time). The Diamond Sūtra is handling all four at once.
What "no mark" actually means
Not that the bodhisattva walks around hallucinating that there is no one else, no time, no distinction. The marks are cognitive constructions; dropping them does not dissolve the phenomena they were constructing around. An analogy: before I read the book, I think of my body as a continuous solid object. After studying cell biology, I still see my body as a continuous solid object — but I now know the apparent continuity is constructed from trillions of cells in constant exchange. The phenomenological continuity is preserved; the ontological claim about self-existent continuity has dropped. The bodhisattva's dropped marks are similar. Other people still appear; interactions happen; time passes. But the cognitive construction treating each of these as self-existent is no longer load-bearing. Other people appear without the imputation that they are "other selves" in the same sense I imagine "myself." Time passes without a persisting "me" laid across it like a beam. This is subtle and requires direct experience to make sense of. Intellectual description reaches a limit here.
The Diamond Sūtra's repeated returns to this point
The Diamond Sūtra does something unusual: it states the four-marks teaching, then restates it, then restates it differently, then returns to it again. In a text of roughly 300 lines, this formulation appears at least seven times. Why the repetition? Because the teaching is easy to miss and easy to partially absorb while leaving the cognitive structure intact. Each return catches a different angle. Section 3 gives the basic statement. Section 6 links it to the raft parable. Section 13 connects it to the "tathāgata who has no fixed form." Section 17 applies it specifically to the bodhisattva's vow. Section 25 returns to the self-mark in the context of whether the Buddha has sentient beings. Section 31 gives the self-mark analysis at maximum technical precision. This repetition is a teaching device. The reader/chanter who hears the statement once has not heard it enough; by the seventh return, if the reader has been actually listening, the statement has a different character than it had at the first. The sūtra's structure is engineered to work on the reader across its length.
Working with the four marks in practice
A 4-week protocol, one mark per week: **Week 1 — Self-mark**. For one week, when you notice yourself saying or thinking "I" (aloud, mentally, implicitly), pause and ask: what am I referring to? A body? A continuous narrator? A persisting personality? See if a definite referent can be found. Expect not to find one; expect the practice of looking to begin to loosen the assumed solidity. **Week 2 — Other-mark**. Same inspection, applied to "other people." When you register another person, notice the cognitive move that treats them as "another self." Ask what this other-self construction is built on — visual features? Prior interactions? Assumed interiority? Inspect without concluding. **Week 3 — Sentient-being mark**. Notice when you group yourself and others into "sentient beings" as a category. This typically activates during generic caring behavior ("I care about all beings"). Inspect: where is the category "sentient beings" located? What distinguishes it from non-sentient stuff? Can the distinction be sharply drawn? **Week 4 — Lifespan-mark**. Notice the sense of your continuity through time. When you plan for next year, or remember five years ago, notice the imputed continuity between the remembering/planning "I" and the "I" who was/will be there. Inspect what the continuity consists of. The inspection is gentle — not trying to disprove the marks, just looking. After four weeks of looking, the marks operate slightly differently. Over years of looking, they operate substantially differently.