Turn on tune in and drop out was the slogan of which proponent of lsd

The correct answer is Timothy Leary.

He used this phrase to encourage people to actively participate in the countercultural movement, which he was a part of.

"Turn on" meant to engage the brain; "tune in" meant to tune into the natural world; and "drop out" meant disengaging from the confines of society. As time went on, this phrase became more about doing drugs than about the actions Leary intended.