The man
Plotinus was born in Egypt around the year 204 and found his teacher in Alexandria, where he stayed for eleven years. Around forty he settled in Rome and taught there for the rest of his life, senators and physicians and women of the court in the room together, no fees, questions welcome. He was famously reluctant about his own person. He refused to sit for a portrait, on the grounds that a painting of the body would be an image of an image, and he would not celebrate his own birthday.
He wrote nothing until his fifties, and what he wrote he left in a heap. His student Porphyry sorted the essays after his death in 270 into six sets of nine, which is all the word Enneads means: the Nines. Porphyry also left us one more report, plainly and without decoration: that during the years he knew him, his teacher arrived at union with the One four times.
The One
Greek philosophy had spent centuries asking what the world is made of and what makes a thing the thing it is. Plotinus pressed the questioning one step further back than anyone before him: whatever exists is one something, so before there can be any thing at all, there must be unity itself. Not a being, however great, but the source that every being borrows its oneness from. He called it the One, and then spent page after page warning that even this name is a courtesy. Calling it One says only what it is not: not many, not divided, not a member of any list.
The One is all things and no one of them.
Everything pours out of it the way light pours from a source that loses nothing by shining. Mind, soul, world, each level a step further out, and none of it ever actually separate. That is his whole cosmology in one image, and he arrived at it with no scripture and no revelation, by argument, in Greek, in Rome.
The return
The outflow is only half of his teaching. The other half is the turn back. Everything that came from the One carries a homesickness for it, and in the soul that homesickness can become a path. Not a journey through space, he insists, because the One was never at a distance. The distance is made of attention pointed the wrong way.
Withdraw into yourself and look.
The Enneads end with a phrase his translator rendered as the flight of the alone to the Alone. For all the architecture of his system, the last step is not a syllogism. Reason walks you to the door, he says, and then you go through it without reason, without images, without even the thought that you are going through. On the far side of the door, there are not two things.
The sculptor
His most famous practical instruction is a sculptor's. A statue is not made by adding marble. The figure is already in the block, and the work is removal: cut away here, smooth there, take off everything that is not the face, and the face shows. Do that with yourself, he says. Never stop working the stone.
By now you know this is the only method anyone on this site teaches. The Upanishads peel with not this, not this. The Tao drops something every day. Eckhart releases. Plotinus chisels. The front page of this site calls it subtraction, and Plotinus is the witness who says most clearly why it works: because what you are cutting toward was in the block the whole time.
The same ground
Plotinus is the Greek hill. He claimed no revelation, quoted no scripture, and reached his report by pushing philosophy past the point where philosophy admits it cannot see. What he described from there matches the Upanishads he had likely never read and a Chinese book he certainly had not: an unnameable source, nearer to you than you are to yourself, reached by removal. And his influence flowed forward into Christian mysticism, shaping the very language Eckhart would inherit a thousand years later. Of all the witnesses, he is the bridge.
Where to begin
Do not start at the beginning; the Enneads are not that kind of book. Start with the essay called On Beauty, the sixth of the first Ennead, where the sculptor lives. The old translation by Stephen MacKenna is out of copyright and free online, and it is a strange, grand piece of English worth reading for its own sake. Take it in small pieces. Plotinus wrote for people who were in the room with him, and the way back into the room is to read him slowly enough to hear the voice.