Is it during deep meditation when the mind is completely stilled that one experiences anatta? Is the conviction in anatta gradual or abrupt? This question would be connected to the 4 stages of enlightenment, too.
Anatta can be seen even by the average Joe. Ex: when there's headache or any kind of bodily pain, why it is not going away at will? When you taste something, why the taste disappear after eating? Why can't you make the taste persist. Why your body stink when you don't take bath? Why can't you stop that from happening? When happiness arises in the mind, why does it fade away without your consent?
I remember back in ~2001 I was very serious about this topic -- trying to experience
Back then I already did ~3 years of the non-attachment practice, as a result I no longer identified myself with my thoughts, but I still could not see how the watcher and the thinker of thoughts was not real.
The way I got first-hand experience of "no-subject" was through meditation on "chitta-vritti" or the train-of-thoughts process. I was watching my thoughts very closely, trying to see where they come from... and how they dissolve...
As I kept watching, I saw that thoughts do not really dissolve, instead they are slowly replaced by the next thought. When I focused on this process of replacement of one thought by the next, I realized that the very "I who is watching this process" was in fact nothing else but the next thought itself(!)
What I clearly saw from this process is that what we normally assume to be the fixed subject of experience is actually transient! Every next thought serves as subject of the previous thought, until it becomes object of the next thought which at that moment is the subject etc.
This is when it became very clear to me that thoughts work by the principle of association. There is nothing like "subject" that "thinks the thoughts" and decides what to do. The mind works by associating the current experience with the memories we had from the previous experiences and coming up with the most appropriate response(s). As I watched myself during day-to-day action, in light of this realization, it became more and more clear to me, that the idea of "I" is something we are trained to overlay on top of the otherwise automatic process. It's a useful trick that helps us explain our choices to other people: "I did X because I wanted Y" or "I said A because I remembered B" - but in reality all this wanting, decision making, doing and thinking - happens automatically through association. The more I watched myself acting, day by day, the more clearly I saw that there was no "I" that acted, the action happened by itself. The more I watched other people, the more I saw that they too respond automatically. (This last realization actually helped me be more effective with people. I stopped expecting them to be objective or reasonable, and accepted that their actions came from their biases and preconceptions.)
An obvious question one can ask at this point: but what about the freedom of will then, is that an illusion? And how do we reconcile this with Buddha's insisting that we must act skillfully and take responsibility? It's alright and still true. Both are valid perspectives on the same reality. Whether automatically or not, acting skillfully still matters. Even if I am an automatic biological robot responding automatically based on my past experience - still, if I make bad decisions, cultivate laziness, sloppiness, bad mental and physical hygiene, let negative mindstates grow - I will suffer the consequences of my actions. Even if they are not truly "my" actions, the experience of results will be "mine". And if I make good decisions, cultivate will power, excellence, good mental and physical hygiene, positive mindstates - there will be experience of those results in due time. Even though the subject of that experience is just the next transient thought and not any fixed "I" - the experience will be real at that point.
So my answer is:
anatta can be directly experienced in meditation on chitta-vritti, by closely watching the train-of-thoughts process. It can then be further clarified by watching one's thoughts and actions objectively in post-meditation, to disidentify from one's opinions, biases, and attachments. Care should be taken to not allow the experience of anatta degenerate into an attitude of fatalism, meekness, and laziness.
Assuming "experience of anatta" means abandoning the fetter of self-view (or sakkāya-diṭṭhi), that and "conviction" (i.e. lack of doubt) sound like "stream-entry".
There's been lots written about stream-entry (or sotāpanna).
Stream-entry is often (usually? always?) portrayed in suttas as being the result of a waking realization, a result of an experience or hearing a dhamma-talk -- one memorable though non-canonical example is Gotami & the Mustard Seed.
I think that (etymologically at least), sakkāya-diṭṭhi is a wrong view about the true or real body -- i.e. it's a view that the aggregates are self.
Note that a "view" is something established, reinforced, believed -- see also How are 'conceit' and 'identity-view' not the same? because the answers there explain the difference between (fixed) "identity view" and a more transient "conceit" -- the eradication of "conceits" should happen at the final stage of enlightenment.
More generally, though, I think the doctrine is that any or every view of self -- including for example "I exist" and "I don't exist" -- is confusing and the result of unwise attention. Maybe that is easy to verify (by inspection).
Understanding Anatta is easy but living it is difficult. I can tell you in which ways you are not experiencing Anatta. If you are identifying body as yourself or as your's then you are not experiencing Anatta. If you are identifying feelings as yourself or as your's then you are not experiencing Anatta. If you are identifying perceptions as yourself or as your's then you are not experiencing Anatta. If you are identifying consciousness as yourself or as your's then you are not experiencing Anatta. If you are identifying volitional formations as yourself or as your's then you are not experiencing Anatta.
In order to experience Anatta you must NOT identify with Body, Feelings , Perceptions,Consciousness and Volitional formations as yourself or as your's. We do that by applying mindfulness. Here I quote from DN 22:
Furthermore, a mendicant meditates by observing an aspect of principles with respect to the five grasping aggregates. And how does a mendicant meditate observing an aspect of principles with respect to the five grasping aggregates? It’s when a mendicant contemplates: ‘Such is form, such is the origin of form, such is the ending of form. Such is feeling, such is the origin of feeling, such is the ending of feeling. Such is perception, such is the origin of perception, such is the ending of perception. Such are choices, such is the origin of choices, such is the ending of choices. Such is consciousness, such is the origin of consciousness, such is the ending of consciousness.’ And so they meditate observing an aspect of principles internally … That’s how a mendicant meditates by observing an aspect of principles with respect to the five grasping aggregates.
Experience of Anatta is not complete unless you have felt revulsion towards body, revulsion towards feelings, revulsion towards perceptions, revulsion consciousness and revulsion towards volitional formations. It is easy to feel revulsion towards body because it decays and dies but it is difficult to feel revulsion towards mind,mentality and consciousness. In order to feel revulsion towards that you must learn dependent origination i.e one thing gives rise to another and cessation of one ceases another. This is beautifully explained in the this sutta.I quote the experience part for your benefit :
“Seeing thus, bhikkhus, the instructed noble disciple experiences revulsion towards form, revulsion towards feeling, revulsion towards perception, revulsion towards volitional formations, revulsion towards consciousness. Experiencing revulsion, he becomes dispassionate. Through dispassion his mind is liberated. When it is liberated there comes the knowledge: ‘It’s liberated.’ He understands: ‘Destroyed is birth, the holy life has been lived, what had to be done has been done, there is no more for this state of being.’”
Everyone are experiencing anattā (5 aggregates), but only the insight-practitioners are understanding anattā as anattā. So, the practitioner is trying to understand "how it is not attā", then he practice to understand like that in everything. But, he doesn't practice to experience anattā. Actually, he practice to experience anatta-lakkhaṇa of anattā (5 aggregates), which he understanding it as attā, by ñātapariññā. Then he practice, tīraṇa-pariññā, that experience in everything to abandon, pahāna-pariññā, atta-vipallāsa in everything.
The deep concentration-meditation can help the insight-practitioners to understand anattā as anattā easier. But the concentration-meditation can not understand anattā by itself, so we need Buddhā to teach us the insight-meditation; we can't practice the insight-meditation before Buddhā enlightened and teach us insight-meditation.
The understanding in anattā as anattā is gradually practice in neyya-practitioner, such as many bhikkhus who gradually memorize a new "little"-sutta in Saṃyuta-nikāya each time after they finished to practice the previous sutta and still not enlighten; but abruptly practice in ugghaṭitaññū-person and vipacitaññū-person, such as Sāriputta and Moggallāna. So, the gradual or abrupt practice are depend on 5 indriya/bala of each practitiner.
Another, this question is not connect to 4 stage of enlightenment, because the sotāpanna already perfectly abandoned atta-saññā/citta/diṭṭhi-vipallāsa, so every ariya have fully anatta-saññā/citta/diṭṭhi without vipallāsa.
So, the question's explanation should be "How to meditate insight meditation", which is explained in most sutta, especially abhidhamma. For the summary:
For the meditation in brief or advance, all already taught in tipitaka and atthakathā. Actually, it is better than my summary above, but you have to successfully meditate the insight-meditation at pa-auk with the tipitaka-memorizer. Then, after you enlightened, you will understand tipitaka&atthakathā easily.
When you comprehend anatta you have to know that all 5 aggregates are within the truth of anatta..And that have to do step by step gradually..The system of meditation Buddha said for that is ''Vipassana'',From that you use to see world as anatta,dukkha and anathma.
when someone use to see things like that trough vipassana meditation,his ten fetters reduce gradually,that means he goes through the 4 stages of enlightenment..
Val… what you have asked is a loaded question, and you cannot expect an answer from another on this. But if you practice ‘dhatu mansikara’ meditation as given below, without any prior expectations of any results from it you will get to the answer to your question. I say this because the conviction on ANATTA comes gradually, and not abrupt. The first stage of Enlightenment and one’s experience of ‘Anatta’ comes before a formal meditation. (See Sankha Kulathantille’s answer.) In formal meditation, you will realize the next three stages. This is how you should go about the ‘dhatu mansikara’ meditation:
A meditator who practices dhatu mansikara focuses his attention on his mind and he begins to perceive his state of mind and thoughts. If the mediator’s attention shifts from the Samadhi state ( state which the meditator perceive his meditation object of respiration) to another sense object, then, he deviates from the state of “being aware” to a state of “expecting something more” from an object. Lets take an example…
Let’s take an individual who practices meditation. Supposing, someone visits him at the time of meditating, he would get irritated and consider this incident to be an hindrance to his meditation practice. He would resist, thinking, “ outsiders always get in my way. It is very difficult to practice meditation with the undue presence of outsiders…”. Why does he get infuriated at this obstruction? His mind has overly embraced his meditation practice; he has excessively attached to the practice; he has locked himself in the practice. In other words, the operation of pathavi element in his mind is so intense. The mind has provided free access (operation of apo) for thoughts to be locked with the obstruction to meditation practice. Due to this freedom present in the mind (operation of apo), thoughts could easily get locked, compressed (operation of pathavi) with the obstruction. As a result of the operation of pathavi element followed by the free access accommodated by the apo element, heat (tejo) begins to rise to a higher degree in the mind. The resultant intense degree of heat generates a similar degree of magnetic energy (kama) in the mind. The higher the degree of heat, the higher the degree of magnetic energy in the mind. The magnetic energy generated in this manner has the potential of making vibrations, movements (in other atoms). As magnetic energy, with the potential of making vibrations, is incorporated and carried in these elements, they are identified as “elements of vayo” (vayo dhatu). I believe that you realize the functioning of the four primary elements, beginning with pathavi dhatu, in the mind.
Even though, we analyze the process of the primary elements with gross examples, when dhatu manasikara (reflection of elements) is practiced in a practical way, a meditator ought to have developed a stable mental state where he could perceive his respiration very clearly. In other words, the meditator needs a strong Samadhi (concentration) to accomplish the practice of dhatu manasikara. The meditator who contemplates the mental phenomena with a strong mindfulness based on the Samadhi, would perceive the changes in the mind on the basis of dhatu manasikara.
The meditator who experinces a blissful mental state, now, begins to catch the first glimpse of the transient nature, and thus the supramundane meaning of anicca of the mind. He begins to observe the changes that take place even in the blissful mental states. He perceives slight differences of bliss he experiences during Samadhi states which give rise to the understanding of the characteristic of dhukka ( altering the current state and transforming to a different state) inherent even in blissful mental states. Thus, a meditator becomes fully convinced of the characteristics of anicca, dukkha and anatta through the reflection of one’s mind. This is the positive outcome, gain that a meditator achieves through the practice of dhatu manasikara.