"Yoniso manasikāro" is defined in the suttas through its function - to develop the skillful qualities:
"Yoniso, bhikkhave, bhikkhu manasi karonto akusalaṃ pajahati, kusalaṃ bhāvetī’’ti.
"A monk who attends appropriately abandons what is unskillful and develops what is skillful."
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"Jānato ahaṃ, bhikkhave, passato āsavānaṃ khayaṃ vadāmi, no ajānato no apassato. Kiñca, bhikkhave, jānato kiñca passato āsavānaṃ khayaṃ vadāmi? Yoniso ca manasikāraṃ ayoniso ca manasikāraṃ. Ayoniso, bhikkhave, manasikaroto anuppannā ceva āsavā uppajjanti, uppannā ca āsavā pavaḍḍhanti; yoniso ca kho, bhikkhave, manasikaroto anuppannā ceva āsavā na uppajjanti, uppannā ca āsavā pahīyanti."
"The Blessed One said, "Monks, the ending of the fermentations is for one who knows & sees, I tell you, not for one who does not know & does not see. For one who knows what & sees what? Appropriate attention & inappropriate attention. When a monk attends inappropriately, unarisen fermentations arise, and arisen fermentations increase. When a monk attends appropriately, unarisen fermentations do not arise, and arisen fermentations are abandoned."
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The specifics of how the attention feeds skillful or unskillful qualities are given in:
Ahara sutta - http://www.accesstoinsight.org/tipitaka ... .than.html" onclick="window.open(this.href);return false;
Ayoniso-manasikara sutta - http://www.accesstoinsight.org/tipitaka ... .than.html" onclick="window.open(this.href);return false;
Titthiya sutta - http://www.accesstoinsight.org/tipitaka ... .than.html" onclick="window.open(this.href);return false;
Silavant sutta - http://www.accesstoinsight.org/tipitaka ... .than.html" onclick="window.open(this.href);return false;
"Monks, there are these two conditions for the arising of right view. Which two? The voice of another and appropriate attention. These are the two conditions for the arising of right view."
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Mulapannasa-Atthakatha 1.64-67 in the words of Bjikkhu Bodhi, in the notes 33 and 36 to the translation of Sabbasava sutta (Pali text added):
Metta, DmytroTattha yoniso manasikāro nāma upāyamanasikāro pathamanasikāro, aniccādīsu aniccanti ādinā eva nayena saccānulomikena vā cittassa āvaṭṭanā anvāvaṭṭanā ābhogo samannāhāro manasikāro, ayaṃ vuccati yoniso manasikāroti.
Wise attention (yoniso manasikāra) is glossed as attention that is the right means (upāya), on the right track (patha). It is explained as mental advertence, consideration, or preoccupation that accords with the truth, namely, attention to the impermanent as impermanent, etc.
Ayoniso manasikāroti anupāyamanasikāro uppathamanasikāro. Anicce niccanti dukkhe sukhanti anattani attāti asubhe subhanti ayoniso manasikāro uppathamanasikāro. Saccappaṭikulena vā cittassa āvaṭṭanā anvāvaṭṭanā ābhogo samannāhāro manasikāro, ayaṃ vuccati ayoniso manasikāroti. Evaṃ yoniso manasikāraṃ uppādetuṃ jānato, ayoniso manasikāro ca yathā na uppajjati, evaṃ passato āsavānaṃ khayo hoti.
Ayoniso manasikāro hi vaḍḍhamāno dve dhamme paripūreti avijjañca bhavataṇhañca. Avijjāya ca sati ‘‘avijjāpaccayā saṅkhārā…pe… dukkhakkhandhassa samudayo hoti. Taṇhāya sati taṇhāpaccayā upādānaṃ…pe… samudayo hotī’’ti.
Yoniso manasikāro pana vaḍḍhamāno – ‘‘yoniso manasikārasampannassetaṃ, bhikkhave, bhikkhuno pāṭikaṅkhaṃ, ariyaṃ aṭṭhaṅgikaṃ maggaṃ bhāvessati, ariyaṃ aṭṭhaṅgikaṃ maggaṃ bahulīkarissatī’’ti (saṃ. ni. 5.55) vacanato sammādiṭṭhipamukhaṃ aṭṭhaṅgikaṃ maggaṃ paripūreti.
Unwise attention (ayoniso manasikāra) is attention that is the wrong means, on the wrong track (uppatha), contrary to the truth, namely, attention to the impermanent as permanent, the painful as pleasurable, what is not self as self, and what is foul as beautiful. Unwise attention, MA informs us, is at the root of the round of existence, for it causes ignorance and craving to increase; wise attention is at the root of liberation from the round, since it leads to the development of the Noble Eightfold Path. MA sums up the point of this passage thus: the destruction of the taints is for one who knows how to arouse wise attention and who sees to it that unwise attention does not arise.
Yasmā pana ime dhammā manasikaraṇīyā, ime amanasikaraṇīyāti dhammato niyamo natthi, ākārato pana atthi. Yenā ākārena manasikariyamānā akusaluppattipadaṭṭhānā honti, tenākārena na manasikātabbā. Yena kusaluppattipadaṭṭhānā honti, tenākārena manasikātabbā.
MA makes the important point that there is no fixed determination in things themselves as to whether they are fit or unfit for attention. The distinction consists, rather, in the mode of attention. That mode of attention that is a causal basis for unwholesome states of mind should be avoided, while that mode of attention that is a causal basis for wholesome states should be developed.