Spiritualists championed a derivative of union theory that asserts that the union, rather than metaphorical, is the literal fusion of two souls (or two half souls) to form one new soul.
In searching for love, we are not just searching for another person with which we have a deep emotional connection, we are searching for a part of ourselves, our “other half”. When we fall in love, we find a soul which completes us, helping us to further understand ourselves in the process. When we fall in love and build a union, we become a shared identity and form a new soul. Both individuals play a central role in forming that soul and new identity.
The whole argument hinges on the idea that humans have souls in the first place. No adequate theory pertaining to the existence of human souls has overcome the challenge of how a non-physical soul can interact with a physical one. Why, for example, when my "soul" wants to move my arm, does my arm move, and not your arm, or the cat's arm? There must be some interaction, but the laws of physics do not permit the interaction between a physical object and a non-physical one. Physics also states that energy cannot be created or destroyed. The laws of momentum also dictate that momentum remains constant in a system. Therefore, a non-physical soul cannot generate momentum, redirect momentum, or influence momentum in the physical world. Without proving the existence of the soul, spiritualists fail to prove the theory of love as two souls coming together. Even if we can accept the existence of a soul, this view of love is constructed around monogamous, binary views of love and sex. It does not extend the existence of love to those in polyamorous relationships. This restrictive and binary view of love is outdated in the modern world where polyamory is increasing and a more progressive attitude towards love is emerging.
[P1] Humans have souls. [P2] When we fall in love, the lovers' two individual, complete (or half) souls merge to form a new complete soul.
[Rejecting P1] Humans do not have souls