There are two approaches to sādhana. One is motivated by an intellectual conception, the other is motivated by an emotional conception. Intellect and emotion are involved in both approaches, but the intellectual approach has intellect as the prime motive and emotion as the supporting instrument, while the emotional approach has emotion as the prime motive and intellect as the supporting instrument.
The intellectual approach is called śāstrik or vaidhik because intellect is born from śāstra (“śāstra yonītvā”). Therefore the intellect-motivated sādhana depends heavily on the depth of ones comprehension of śāstra, especially the depth of the comprehension that all the conclusions point towards bhakti (love) being the supreme morality, the supreme rule, the supreme goal, and the Divine (Viṣṇu) being the supreme beloved.
Clear knowledge of these conclusions of śāstra is also essential in the emotionally-motivated approach to sādhana – because intellectual conceptions are important supporting instruments of the sādhana. The primary requirement, however, is the force, clarity and specificity of the emotional attraction to Divine Love, bhakti.