H. P. Blavatsky

Spiritualism. In philosophy, the state or condition of mind opposed to materialism or a material conception of things. Theosophy, a doctrine which teaches that all which exists is animated or informed by the Universal Soul or Spirit, and that not an atom in our universe can be outside of this omnipresent Principle — is pure Spiritualism. As to the belief that goes under that name, namely, belief in the constant communication of the living with the dead, whether through the mediumistic powers of oneself or a so‐called medium — it is no better than the materialisation of spirit, and the degradation of the human and the divine, souls. Believers in such communications are simply dishonouring the dead and performing constant sacrilege. It was well called “Necromancy” in days of old. But our modern Spiritualists take offence at being told this simple truth.

Source: H. P. Blavatsky, The Theosophical Glossary, art. Spiritualism