And if one should ask: Now I am convinced that the S'firot exist, but what should compel me [to think] that there are ten of them but they act as a single force?
Ten Emanations Explained
Beur Eser Sefirot 4:1
Beur Eser Sefirot 4:1
And if one should ask: Now I am convinced that the S'firot exist, but what should compel me [to think] that there are ten of them but they act as a single force?