Humphry Davy on laughing gas (nitrous oxide):

"I lost all connection with external things; trains of vivid visible images passed through my mind and were connected with words in such a manner as to produce perceptions perfectly novel. I existed in a world of newly connected and newly modified ideas … I exclaimed to Dr Kinglake, “Nothing exists but thoughts!” …

I have often felt very great pleasure when breathing it alone, in darkness and silence, occupied only by ideal existence."