The Svacchanda Tantra

Acharya Utpala had likely experienced this bliss of Shiva-advaita non-dualism of Shiva himself during his long life1—

Even sorrows become joys, and even poison becomes nectar.
That path where the world itself becomes liberation is the path of Shankara referring to Lord Shiva. (20.12)

Those who are satisfied by the nectar flowing from the realized form of the Lord,
Having uprooted their thirst, they wander intoxicated as they please. (12.4) (Shivastotravali)

All the above details may be considered merely an introductory exhortation to the Kashmiri Pratyabhijna-Shaiva philosophy. The theoretical elaboration of the Pratyabhijna philosophy is available in its entirety in Utpala’s timeless works: Shivastotravali, Ishvara-pratyabhijna-karika, Ishvara-pratyabhijna-karika-vritti, Ishvara-pratyabhijna-vivriti (commentary), the Siddhitrayi (Ajadapramatrisiddhi, Ishvarasiddhi, Sambandhasiddhi), and the Shivadrishti-vritti. Together, Acharya Utpala’s first three texts and the Vimarshini and Vivriti-vimarshini written upon them by Acharya Abhinava constitute the Shastra-panchaka the five authoritative texts of the Pratyabhijna philosophy.

In the Pratyabhijna philosophy, the Supreme Lord is considered endowed with the powers of consciousness (Chit), bliss (Ananda), will (Iccha), knowledge (Jnana), and action (Kriya), and is the perpetual doer of the five acts: creation, sustenance, dissolution, concealment, and grace—

Shiva is the Self alone, pulsating in all states as the omnipresent consciousness of bliss,
With an unstoppable expansion of will and expanding knowledge and action. (Shivadrishti, 1.2)

In this philosophy, the evolution of the 36 tattvas principles from Shiva to Earth, the 118 worlds, all gross and subtle states, and the seven knowers from Shiva to the individual soul, is described as originating from a single Supreme Reality. Bondage and liberation, the living-liberated state, and the relation between Shiva and Shakti are all described as the divine play of the one Supreme Lord. Consequently, just like the "Brahman is real" or the doctrine of Brahma-advaita non-dualism of Brahman in Shankara-Vedanta, this philosophy establishes Shiva-advaita. Utpala Deva himself calls this philosophy Advayavada non-dualism.

1. I am foolish, I am afflicted by sorrow, I am afraid of defects like old age,
I am devoid of power, I have taken refuge in You.
O Shambhu, act quickly so that I may attain,
The supreme goal, having abandoned the path of suffering. (Shivastotravali, 11.8)