The Buddha compares the reasons for the success or failure of a shopkeeper to the reasons for the success or failure of a bhikkhu's attainment of serenity.
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Index to available translations: AN 3.019
Good advice for businessmen - get down to business! - and good advice for those practicing samadhi.
The advice for bhikkhus turns on the understanding of the phrase:
sakkaccam samadhinimittam adhitthati:
"sincerely bear down on the sign or mark of serenity".
Woodward: "concentrate on the mark of his meditation exercise";
and Bhk. Bodhi: "dilligently apply himself to an object of concentration".
Putting aside the confusion we have here over what is being translated 'concentration' [Woodward: adhitthati; Bhk. Bodhi: samadhi], in this case this 'nimitta' has nothing to do with either a 'mark of meditation exercise' or 'an object of concentration' (where in both cases the translators understand the meaning of nimitta to be the 'reflex-image' of a concentration device).
The word 'samadhinimittam'
[PED has: 2. Description and characterization of samadhi: Its four nimittas or signs are the four satipatthanas M I.301]
is a compound, which following Woodward and Bhk. Bodhi's understanding would necessitate the translaltion: 'samadhi-reflex image', or 'meditation-reflex image', or 'concentration-reflex image' meaning some sort of reflex image of the state one is attempting to achieve which would require it to be had before the thing being done to acquire it.
The term 'reflex-image' is used by the translators of the commentary. I don't know what Pali word is being so translated or the precise description of the process being recommended, but the risk is that this will be understood as some sort of 'after-image' - something like when concentrating on a red splotch one looks up and sees a green splotch.
As that term is being translated and explained it means the image of the kasina or concentration device or object useful for establishing concentration, calm or serenity or detachment; that is, the image itself, and the object is to get, cultivate and maintain this image 'in the mind's eye.'
That is not what is being looked for in this situation.
The idea here is 'sign' or 'indication' of samadhi (serenity).
Something indicating the way access to serenity has been previously attained or to be watched for if it has not been previously attained.
The thing to be looked for here is the state of having been absorbed directly (fascinated, interested ... don't get hung up on the term!, it's the 'directly' part that's important - without the usual intervening awareness of awareness) in following the breathing, a thought (vitakka or vicara) or in some day-dream-like image, a clear mental picture of something one is working on, planning etc. or of a concentration device (kassina), ... and it is not the object (the image or the thought) that is the sign to be noticed, it is the direct absorbtion, the viewing of the object or situation or observing of the train of thought without self-conscousness (self-awareness, an intervening awareness of awareness) that is the nimitta, the sign or indication of the onset of serenity.
At first becoming aware of this sign will interrupt the processs.
That is why one should review the stages after one's practice.
This is yonisomanisikaro: (studious etiological examination) trace out in your mind the features of the point of onset, what factors contribute to the maintenance of the serene state, and what brings it to an end.
Over time (the practice is to go back and forth between self-conscious observance of the image and the absorbed state: being absorbed, loosing the absorbed state, bringing the mind back to the image) one will loose self consciousness and allow progress.
That is achieved by not hanging on to the object and allowing the process to unfold on it's own.
Part of one's careful planning here is to remember to include a command (a mental program) to emerge from this direct absorption: "Let me emerge from this absorption by becoming aware of self-awareness and worldly objects [at such-and-such a time, or after so long, or upon such and such an event, i.e., nightfall, dawn.]" Without taking the precaution of an exit strategy one is highly likely to experience only flashes of the experience of samadhi, or not to enter it in the first place.