Bhagavad Gita Chapter 18, Verse 13

पञ्चैतानि महाबाहो कारणानि निबोध मे | साङ्ख्ये कृतान्ते प्रोक्तानि सिद्धये सर्वकर्मणाम् ||

pañcaitāni mahā-bāho kāraṇāni nibodha me sāṅkhye kṛtānte proktāni siddhaye sarvakarmaṇām

O mighty-armed one, know these five causes for the accomplishment of all actions, as declared in the Sankhya philosophy at the conclusion.

Krishna introduces the teaching about the five causes of action. 'Pañcaitāni mahā-bāho kāraṇāni nibodha me'—O mighty-armed one (mahā-bāho), know (nibodha) these five causes (pañca etāni kāraṇāni) from Me (me). 'Sāṅkhye kṛtānte proktāni'—they were declared (proktāni) in Sankhya philosophy (sāṅkhye) at the conclusion (kṛtānte). 'Siddhaye sarvakarmaṇām'—for the accomplishment (siddhaye) of all actions (sarvakarmaṇām). This sets up the teaching about the five factors that contribute to every action: the body, the doer, the various instruments, the different kinds of effort, and the Supreme. Understanding these helps you see that action is complex, involving multiple factors, and that you're not the sole doer. This prepares for the teaching about the three kinds of action according to gunas that will follow.