SB 3.5.29

mahat-tattvād vikurvāṇād
 ahaṁ-tattvaṁ vyajāyata
kārya-kāraṇa-kartrātmā
 bhūtendriya-mano-mayaḥ
vaikārikas taijasaś ca
 tāmasaś cety ahaṁ tridhā
Synonyms: 
mahat — the great; tattvāt — from the causal truth; vikurvāṇāt — being transformed; aham — false ego; tattvam — material truth; vyajāyata — became manifested; kārya — effects; kāraṇa — cause; kartṛ — doer; ātmā — soul or source; bhūta — material ingredients; indriya — senses; manaḥ-mayaḥ — hovering on the mental plane; vaikārikaḥ — the mode of goodness; taijasaḥ — the mode of passion; ca — and; tāmasaḥ — the mode of ignorance; ca — and; iti — thus; aham — false ego; tridhā — three kinds.
Translation: 
Mahat-tattva, or the great causal truth, transforms into false ego, which is manifested in three phases — cause, effect and the doer. All such activities are on the mental plane and are based on the material elements, gross senses and mental speculation. The false ego is represented in three different modes — goodness, passion and ignorance.
Purport: 

A pure living entity in his original spiritual existence is fully conscious of his constitutional position as an eternal servitor of the Lord. All souls who are situated in such pure consciousness are liberated, and therefore they eternally live in bliss and knowledge in the various Vaikuṇṭha planets in the spiritual sky. When the material creation is manifested, it is not meant for them. The eternally liberated souls are called nitya-muktas, and they have nothing to do with the material creation. The material creation is meant for rebellious souls who are not prepared to accept subordination under the Supreme Lord. This spirit of false lordship is called false ego. It is manifested in three modes of material nature, and it exists in mental speculation only. Those who are in the mode of goodness think that each and every person is God, and thus they laugh at the pure devotees, who try to engage in the transcendental loving service of the Lord. Those who are puffed up by the mode of passion try to lord it over material nature in various ways. Some of them engage in altruistic activities as if they were agents appointed to do good to others by their mental speculative plans. Such men accept the standard ways of mundane altruism, but their plans are made on the basis of false ego. This false ego extends to the limit of becoming one with the Lord. The last class of egoistic conditioned souls — those in the mode of ignorance — are misguided by identification of the gross body with the self. Thus all their activities are centered around the body only. All these persons are given the chance to play with false egoistic ideas, but at the same time the Lord is kind enough to give them a chance to take help from scriptures like Bhagavad-gītā and Śrīmad-Bhāgavatam so that they may understand the science of Kṛṣṇa and thus make their lives successful. The entire material creation, therefore, is meant for the falsely egoistic living entities hovering on the mental plane under different illusions in the modes of material nature.