Philosophy which does not aim for devotional service to the Lord is considered as mental speculation.
Bhakti-yoga, devotional service, is the basic principle of all systems of philosophy; all philosophy which does not aim for devotional service to the Lord is considered merely mental speculation. But of course bhakti-yoga with no philosophical basis is more or less sentiment. There are two classes of men. Some consider themselves intellectually advanced and simply speculate and meditate, and others are sentimental and have no philosophical basis for their propositions. Neither of these can achieve the highest goal of life–or, if they do, it will take them many, many years. Vedic literature therefore suggests that there are three elements–namely the Supreme Lord, the living entity and their eternal relationship–and the goal of life is to follow the principles of bhakti, or devotional service, and ultimately attain to the planet of the Supreme Lord in full devotion and love as an eternal servitor of the Lord.
Sankhya philosophy is the analytical study of all existence. One has to understand everything by examining its nature and characteristics. This is called acquirement of knowledge. But one should not simply acquire knowledge without reaching the goal of life or the basic principle for acquiring knowledge–bhakti-yoga. If we give up bhakti-yoga and simply busy ourselves in the analytical study of the nature of things as they are, then the result will be practically nil. It is stated in the Bhagavatam that such engagement is something like husking a paddy. There is no use beating the husk if the grain has already been removed. By the scientific study of material nature, the living entity and the Supersoul, one has to understand the basic principle of devotional service to the Lord.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Third Canto, Chapter 29 – Text 02
