The entire universe can be divided into 2 main categories: Prakriti and Purusha.
Prakriti is matter + energy. Prakriti is limited by time, space and form. Prakriti functions on cause and effect, which popularly people call Karma. Karma is the principle of cause and effect. Wherever there is a cause, there will be an effect.
Purusha is the underlying principle that is differentiable from Prakriti. There is something that is beyond time and space in this universe that we as embodied beings are able to access and experience. It isn’t a physical entity and so formless. And hence not a “thing”. It has no gender. Since it is out of space and time, it is not even an “it”. It is experienced as dynamic-stillness. It moves everything, but doesn’t seem to move. From Yoga’s point of view, the power of Prakriti, that is, the mind, the intellect, the body and consciousness itself receive their light from this principle. To give a simple explanation, Purusha is to the universe, what electricity is to a light bulb.
Science studies Prakriti objectively devoid of self-study and introspection. That is a philosopher’s job. Yoga says no to this approach. According to Yoga, the material world cannot be separated from the one who observes it. Moreover, without self-knowledge, all observations will be conditioned and coloured by one’s past. So yoga seeks to uncondition and free the observer from his past, which colours perception. This is done through transformatory efforts, introspection and very importantly wisdom. Yoga studies both the subject (the observer) as well as the object (the observed) all the while urging us to practically experiment and explore to understand that thought (citta vrtti) creates a separation between the subject and object. So when citta-vrtti nirodha happens, the subject-object split dissolves, which is the definition of Yoga as given by Patanjali. This happening is called Samadhi.
Since Prakriti is within the realm of space and time, it is impermanent and modifiable. We can look within to understand this. Everything that is impermanent within us falls under Prakriti. Body, vitality, physiology, biochemical processes, neurological movements, feelings, emotions, thoughts, states of mind such as sadness and happiness. All of these are impermanent and modifiable. Yoga recognises this aspect of Prakriti as an opportunity. It first works to bring some harmony within this dimension, which is still within the reach of voluntary action. This is where self-discipline (asana, pranayama, ahara, nidra) and inquiry (vitarka), probing and questioning (vicara), play an important role.