Verse 2.17

द्रष्टृदृश्ययोः संयोगो हेयहेतुः ॥१७॥

draṣṭr̥-dr̥śyayoḥ saṁyogo heyahetuḥ ॥17॥

The cause of that which is to be avoided is the union of the Seer(subject) and the seen(object).

Verse 2.18

प्रकाशक्रियास्थितिशीलं भूतेन्द्रियाअत्मकं भोगापवर्गार्थं दृश्यम् ॥१८॥

prakāśa-kriyā-sthiti-śīlaṁ bhūtendriya-ātmakaṁ bhoga-apavarga-arthaṁ dr‌śyam ॥18॥

The experienced is composed of elements and organs, is of the nature of illumination, action and intertia, and is for the purpose of experience and release (of the experiencer).

Who is the seer? 

The Self of Man, the Purusa.

What is the seen? 

The whole of nature, beginning with the mind, down to gross matter. 

All this pleasure and pain arises from the junction between this Purusa and the mind. The Purusa, is pure; it is when it is joined to nature, and by reflection, that it appears to feel either pleasure or pain.

The visible objective world consists of elements of nature and senses of perception comprising three qualities or attributes (gunas) , which are illumination, motion or action, and inertia or dormancy. 

All these exist eternally to serve the seer (the subject) for the purpose of experiencing the pleasures and objects of the world, or for liberation

This sutra describes the characteristics, actions and uses of nature (prakrti) .

The three attributes of nature are sattva, rajas and tamas.

According to Patanjali, sattva, rajas and tamas represent prakasa, kriya and sthiti. 

These attributes have their own virtues for example, prakasa or brilliance or splendour is sattva;

kriya or study, investigation and action is rajas; and the essence of the being resting as sthiti or dormancy is tamas.

All these attributes and virtues are established in the elements of nature, senses, mind, intelligence and ego. Together they function harmoniously in the form of illumination, action and inertia, allowing the seer to enjoy the world's pleasures (bhoga) ; or by divesting himself of them, to experience liberation.

PATANJALI YOGA SUTRAS