Verse 18

saadhanaanyatra chatvaari kathitaani maniishhibhiH .
yeshhu satsveva sannishhThaa yadabhaave na sidhyati

In this, the wise have spoken of four preliminary requisites. the experience of th Real is possible only if they exist and impossible in their absence.

In this sloka Sri Bhagavatpaada teaches by processes of anvaya and vyatireka: agreement by presence and agreement in absence - that possession of of the four fold means only qualifies for inquiry into Brahman.

In the world, if, when something exists there is th non-existence of something else, that something else cannot be the cause of that thing. To put it another way, when in the non-existence of a thing, some thing exists, th latter cannot be the cause of t he former. When a cloth has come into existence in the absence of clay, the clay cannot be the cause of the cloth.

Applying this to viveka and to brahmajijn~aasaa (the desire to know Brahman), where there is no jignjn~aasaa, there will not be viveka etc. There may be things other than viveka etc. Hence it becomes clear that viveka etc. are the cause of brahmajijn~aas aa. There fore, brahmajijn~aasaa on which depends brahmanishtaa will arise only when aided by the saadhana catustaya begining with viveka, and will not arise in their absence, it is established that these requisitesare preliminary to barhmajijn~aasaa whic h has as its goal what is known as Brahmanishtaa.

maniisibhih kathitaani: manIsitvam, the quality of being a manIsin means having knowledge of the import of the sruti.

Many sruti texts support Sri Bhagavatpaadaa's enumeration of saadhana catustaya, and the requirement of nityaanityavastuviveka. The saadhanacatustaya are enumerated as saanto daanta uparatastitiksuh samaahitassraddaavitto bhUtvA AtmanyevAtmAnam pasyet (B rh.)

kathitaani: are said to be. The Upanishads convey the knowledge of Brahman which is the supreme object of human quest, which cannot be achieved by other means of knowledge and which cannot be sublated in any of the three periods of time. The qualificatio n for such jijn~aasaa accrues only to one who has the qualities mentioned successively in vivekAdi (i.e., viveka etc.).