Being is spiritual because it is the activity of the divine Mind, God. Taking place in and as consciousness, and single in quality and quantity because it has one infinite source, or Principle, being does not permit anything inconsistent with itself. "For God to know, is to be; that is, what He knows must truly and eternally exist," No and Yes, p. 16. Mrs. Eddy writes.
Nothing exists outside Mind's self-containment, nor is there any source from which an additional element could come or any place to which it could go. Divine Mind knowing itself and its own ideas constitutes the only true selfhood there is: God, Mind, is the Ego in whom all real being unfolds. No wonder Mrs. Eddy could write: "Nothing can be novel to eternal Mind, the author of all things, who from all eternity knoweth His own ideas. Deity was satisfied with His work. How could He be otherwise, since the spiritual creation was the outgrowth, the emanation, of His infinite self-containment and immortal wisdom?"Science and Health, p. 519.
In the Bible we read, "The Lord knoweth them that are his."II Tim. 2:19. Divine Mind knows the ideas it includes. It could not know anything else but its ideas, nor, in reality, is there anything else for Mind to know. This means that, as we accept the Mind that is God as our Mind, our only legitimate experience is as Mind's ideas.