svaptavyam apy eva vicakShaNena
kaaya-klamo v"api niShevitavyaH
na tv eva saMcintyam a-san nimittaM
yatr' aavasaktasya bhaved an-arthaH
A clear-sighted person should even sleep
Or resort to physical exhaustion,
But never dwell on a bad stimulus
The reaction to which would be adverse.
This verse can be seen as including a kind of definition of what a bad stimulus is. A stimulus is bad if it produces an adverse reaction.
In discussing what is a bad stimulus it might not always be necessary to speak in terms of absolutes, pronouncing "That stimulus is bad, false, untrue. You who use that stimulus are non-Buddhists. We who do not use that stimulus are true Buddhists." That kind of expression might be only an expression of our own deep-rooted ignorance.
No, what makes a stimulus bad is a relative matter. For one who is suffering from flu, to think about the vegetable patch that needs digging might be a very bad stimulus. For one whose mind is crowded with abstract thoughts, I can report from recent experience, to go out and dig the garden might not be a bad idea at all.
For a diabetic, refined sugar is a terribly bad stimulus. But for a child on a beach on a hot day, with salty air blowing in from the sea, eating an ice cream might not produce any kind of adverse reaction at all. It might be more a case of a little bit of what you fancy does you good.
That said, abstract or fanciful thoughts do have a strong generic tendency to fall into the bad stimulus category. So the bad stimulus of this verse and the unhelpful thought of the previous verse might be the same bad stimulus, or the same unhelpful thought. It might be an optimistic expectation or a pessimistic worry; or it might be a denial of cause and effect along the lines of "Why me?" or "It's not fair"; or it might be a sexual fantasy, or a malicious desire for revenge. It might be a misconception about correct posture or breathing. It might be the unreasonable expectation of being able to do an undoing. It might be wishful thinking for enlightenment as an end, by a person who has not bothered himself much with attending to the proper means. It might be the thought, rooted in fear, that "I am right. I am on the side of the righteous. I am one of the good guys. I am a true Buddhist."
An adverse reaction might mean for example fixing or slumping, or getting stuck in greed or anger or ignorance.
The prudent man should even sleep or undergo bodily fatigue; but in no case should he meditate on wrong subjects, by attachment to which disaster might ensue.
A man of clear vision could even resort to sleep, or to physical exhaustion; but he should absolutely not meditate on a bad subject, dependence on which might bring negative consequences.
svaptavyam = nom. sg. m. of gerundive from svap: to sleep
vicakShaNena = inst. vicakShaNa: conspicuous , visible , bright , radiant , splendid; distinct ; clear-sighted (lit. and fig.) , sagacious , clever , wise , experienced or versed in
klamaH = nom. sg. m. klama: fatigue , exhaustion , languor , weariness
api: also, even
niShevitavyaH (nom. sg. m. gerundive from niShev): to be resorted to, to be practised
samcintyam (nom. sg. n. gerundive from sam-cint): to be thought over or considered
sam-cint: to think about, think over, consider carefully, reflect about; to design, intend, destine
asat (nom. sg. n.): untrue , wrong; bad; n. untruth , falsehood
nimittam (nom. sg.): n. cause, stimulus
yatra: in/on which
avasaktasya = genitive of avasakta: suspended from , attached to (as to the shoulder or to the branch of a tree &c ) , bound round ; being in contact with ; belonging
bhavet = 3rd person singular, optative of bhuu: to be
anarthaH (nom. sg.): m. disappointing occurrence , reverse , evil