Shiva is Prakāśa, which is explained as the Light of Consciousness or The Absolute. The entire universe is pervaded by Prakāśa and nothing exists without being Prakāśa. In other words, without Shiva, nothing exists; so is the case with all individual souls, also known as jīvātman. Prakāśa is self-illuminating and it does not depend upon anything for its illumination. Kaṭha Upaniṣad (II.ii.15) confirms this. “In the presence of Brahman, the sun does not shine, nor do the moon and stars, nor does lightning, let alone this fire. When Brahman shines, everything follows. By Its light, all these are lighted.”
na tatra sūryo bhāti na candratārakaṃ
nemā vidyuto bhānti kutoyamagniḥ।
tameva bhāntamanubhāti sarvaṃ
tasya bhāsā sarvamidaṃ vibhāti॥
न तत्र सूर्यो भाति न चन्द्रतारकं
नेमा विद्युतो भान्ति कुतोयमग्निः।
तमेव भान्तमनुभाति सर्वं
तस्य भासा सर्वमिदं विभाति॥
There cannot be any variety apart from the Supreme Consciousness (Shiva/ Prakāśa), as everything including time and space as their essential nature is only Shiva (omnipresent). Thus, in the whole universe there is only one entity, which is Prakāśa, which is also known as saṁvid (consciousness, intellect, knowledge, understanding).
Prakāśa is intrinsic light, with which an existence is known without the use of antaḥkaraṇa (mind, intellect and ego) and senses. No external light is needed to realise this Light, because it is intrinsical in nature. Only because of Prakāśa (Shiva), other mental and physical entities are made known to us. (Hence Shiva is also known as the cause of creation (Creator), as without Him, nothing exists). Only this Prakāśa manifests as many (effect of creation/cause), which is nothing but the reflection of this Prakāśa everywhere. Thus, there is only one Prakāśa and there is no second to that. It is free from any type of limitation (having all appearances, and at the same time having no appearance; hence Shiva is called indescribable.)
Shiva or Prakāśa has five types of intrinsic powers and He alone has these Absolute Powers and no one else. Hence it is called Svātantrya Śakti (Absolute and Independent Power) and this power is Śakti (Vimarśa). Cit Śakti (Consciousness) and Ānanda Śakti (Bliss) are the two essential powers of Shiva. This is explained as Saccidānanda in Vedānta (evam saccidānanda svarūpaṁ ātmānaṁ vijānīyāt एवम् सच्चिदानन्द स्वरूपं आत्मानं विजानीयात् – Tattvabodha). His other three śakti-s – icchā, jñāna and kriya śakti are known as His power of will, power of knowledge and power of becoming each and every form (omnipresent). His power of pure consciousness is known as Cit Śakti (Citśakti), His transcendental (supernatural) aspect and His power of Bliss is known as Ānanda Śakti, which is nothing but the essential nature of pure Consciousness. Thus both Cit Śakti and Ānanda Śakti (Śiva and Śakti) constitute Lord Shiva (hence Śiva and Śakti are described as inseparable). The other three śakti-s – icchā, jñāna and kriya śakti-s are used for the purpose of creation, etc.
The individual soul or ātman is nothing but Shiva Himself. But His true nature of Prakāśa is concealed by His own power called māyā śakti in the form of a veil. Māyā śakti can be compared to the dark clouds covering the sun and Prakāśa Shiva can be compared to the bright sun. When dark clouds around the sun move away, the real nature of sun is revealed. Similarly, when the darkness of māyā is removed through sādhana (practice such as mantra japa and meditation), Prakāśa Shiva reveals Himself. This is Self-realization.
Shiva has five aspects known as Īśāna, Tatpuruṣa, Sadyojāta, Vāmadeva and Agohora. These five aspects of Shiva are represented by the five śakti-s - Cit Śakti, Ānanda Śakti and icchā, jñāna and kriya śakti-s respectively.