Context: John Donne says that dull sub-lunary lovers–that is, earthly lovers, in contrast to his love and him, who are spiritual, refined lovers–cannot tolerate separation; but he and his love care less than ordinary people about physical contact. Their two souls are, through the alchemy of love, actually only one. If their souls are actually two, they are like the legs of a pair of compasses, which are seemingly two but are joined in such a fashion as to be one. If he must part from her to roam, he moves freely, as the outer leg of a pair of compasses moves; she, the central or fixed leg of the compasses, apparently does not move at all, but actually moves in sympathy with the other one. Her soul yearns after his and moves with his, although not as apparently as his does. Her firmness is an anchor for him and makes him come back to his starting point. The symbolism of the circle indicates that she enables him to complete himself, for the arc is a sign of fragmentation, but the circle a sign of completion and perfection. Donne says:
If they be two, they are two so
As stiff twin compasses are two;
Thy soul, the fixed foot, makes no show
To move, but doth, if th'other do.