Dhīvaśātsattvasiddhiḥ धीवशात्सत्त्वसिद्धिः॥ (sūtra III.12)

Dhī – the highest level of spiritual thought process; vaśāt – by means of; sattva – realization of the illuminating Self within; siddhiḥ - complete attainment.

When the aspirant develops qualities hitherto described, he acquires the highest level of spiritual intellect and by means of which he realizes Self illuminating Śiva within. Sattva in this aphorism does not refer to one of the three guṇa-s. It is a single world used to refer realization of the Self within. Because of the aspirant having developed all those qualities discussed earlier leading to the highest knowledge of spiritual intellect by cleansing his mind, which becomes devoid of dualities due to sealing of his sensory inputs, he is able to reach the stage of complete attainment by realizing the Self-illuminating Śiva within. Dhi, the highest level of spiritual thought process leads to spiritual intuition, which unfolds in his mind, as his mind is now totally sublimated. 

Developing spiritual intuitive power is important, as Divine commandments are passed on to the aspirant through intuitive thought process. When an aspirant reaches irreversible stage of spiritual progression, he is guided to the highest stage only through the commandments received in his mind. Normally, great sages and saints are directed to guide such an aspirant to the level of Śaktī who finally takes him to Śiva for final emancipation.

This aphorism says that Self-realization is possible, only if one is able to develop the highest level of spiritual intellect. In other words, when his mind is totally cleansed from sensory addictions, he is able to reach logical spiritual goal of complete attainment, where he realizes Śiva in His true nature of Self-illumination.