SB 7.7.47

karmāṇy ārabhate dehī
 dehenātmānuvartinā
karmabhis tanute deham
 ubhayaṁ tv avivekataḥ
Word for word: 
karmāṇi — material fruitive activities; ārabhate — begins; dehī — a living entity who has accepted a particular type of body; dehena — with that body; ātma-anuvartinā — which is received according to his desire and past activities; karmabhiḥ — by such material activities; tanute — he expands; deham — another body; ubhayam — both of them; tu — indeed; avivekataḥ — due to ignorance.
Translation: 
The living entity, who has received his present body because of his past fruitive activity, may end the results of his actions in this life, but this does not mean that he is liberated from bondage to material bodies. The living entity receives one type of body, and by performing actions with that body he creates another. Thus he transmigrates from one body to another, through repeated birth and death, because of his gross ignorance.
Purport: 

The living entity’s evolution through different types of bodies is conducted automatically by the laws of nature in bodies other than those of human beings. In other words, by the laws of nature (prakṛteḥ kriyamāṇāni) the living entity evolves from lower grades of life to the human form. Because of his developed consciousness, however, the human being must understand the constitutional position of the living entity and understand why he must accept a material body. This chance is given to him by nature, but if he nonetheless acts like an animal, what is the benefit of his human life? In this life one must select the goal of life and act accordingly. Having received instructions from the spiritual master and the śāstra, one must be sufficiently intelligent. In the human form of life, one should not remain foolish and ignorant, but must inquire about his constitutional position. This is called athāto brahma-jijñāsā. The human psychology gives rise to many questions, which various philosophers have considered and answered with various types of philosophy based upon mental concoction. This is not the way of liberation. The Vedic instructions say, tad-vijñānārthaṁ sa gurum evābhigacchet: to solve the problems of life, one must accept a spiritual master. Tasmād guruṁ prapadyeta jijñāsuḥ śreya uttamam: if one is actually serious in inquiring about the solution to material existence, one must approach a bona fide guru.

tad viddhi praṇipātena
 paripraśnena sevayā
upadekṣyanti te jñānaṁ
 jñāninas tattva-darśinaḥ

“Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized soul can impart knowledge unto you because he has seen the truth.” (Bg. 4.34) One must approach a bona fide spiritual master by surrendering himself (praṇipātena) and rendering service. An intelligent person must inquire from the spiritual master about the goal of life. A bona fide spiritual master can answer all such questions because he has seen the real truth. Even in ordinary activities, we first consider gain and loss, and then we act. Similarly, an intelligent person must consider the entire process of material existence and then act intelligently, following the directions of the bona fide spiritual master.