It has been heard that without Him nothing can move. Here, it is not only referred Easwaran as He but also She. She is integrated with Easwaran. He and She is one. Hence, we call Him as Arthanareeswara.
Easwaran = Light; All other aspects are Ambal
Siva is the combination of two that is He and She. If Siva is to be manifested Easwari is needed. The idol of Sivalingam is not carved out either of male or female alone. But it is carved out of unified stone of male and female aspects. Thousand years ago during the reign of Pallava, there was no separate temple for Ambal. Because, wherever Easwaran is there is Ambal. But, the successive generations gave form to Ambal and built temples to worship her.
Brahma can create only if Sarawathi exists. If Mahalakshmi is not in the heart of Narayana, he can’t move. Once, Balaji was in trance without knowing who he is when Mahalakshmi was not with him. When she came as Kamadhenu and gave milk to Balaji, he began to come out from the trance and knew himself. Thus, it is true that He is within Her.