पञ्चैतानि महाबाहो कारणानि निबोध मे। सांख्ये कृतान्ते प्रोक्तानि सिद्धये सर्वकर्मणाम् ॥

pañchaitāni mahā-bāho kāraṇāni nibodha me sānkhye kṛitānte proktāni siddhaye sarva-karmaṇām

These five causes, O mighty-armed, learn of Me as declared in the Sâñkhya system for the accomplishment of all actions:

In plain words: Krishna tells Arjuna to learn from him the five causes behind every action. These come from Sankhya, an ancient school of thought.