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You will often see many transhumanists try to defend their beliefs by claiming that your opposition to transhumanism is unscientific or grounded in religion. Well, you'd be surprised to see that they live in an illusion that their views are anything but scientific.
Transhumanism, with its belief that the human mind can exist outside of a human body, finds its basis in dualism, a philosophy which posits that essence (in case of humans, "soul", "mind", "spirit" or various other terms) and matter are fundamentally separate and can coexist independently of each other. This is directly taken from Descartes, who was reluctant to abandon his faith and clung to some basics of the Christian dogma, such as the existence of a soul, which he claimed was the same as the mind. Nowadays it's quite an outdated view, especially due to various scientific facts that have been discovered since - above all, the consequences of brain damage. According to Cartesian dualism, the mind is transcendental, non-material and eternal, but damage to the brain (a damage to matter) can cause various effects on a person's mind, such as ill effects to their mental well-being or a sudden change in personality, which completely debunks that theory.
The opposite of dualism is monism, which presumes that essence and matter are one and the only substance and ties the mind to biology; the priniciples of contemporary science tend towards monism. This is the reason why most of the mainstream scientific community looks down on transhumanism and transhumanists, some even going as far to call it a "cult" and a "Rapture for nerds". This isn't far from the truth, as transhumanism is an anachronistic philosophy basing itself on a purely metaphysical principle and burdened by religious legacy, and nothing but a futile attempt to avoid accepting the meaninglessness, absurdity and shortness of life.
It is impossible. Face it.