What is conceptual art...

Sol LeWitt wrote in 1970:

When an artist uses conceptual form of art, it means that all the planning and the decisions are made beforeand, and the execution is a perfuncotry affair. The idea becomes the machine which makes art.

What did he mean with this sentence?
I abstracted it from the context and I started reflecting on it. What there is behind the conceptual art. The idea, the concept.
But is art only the "idea", or also the way of expressing it. I don't have an answer.
Is it true that the execution is a perfuncotry affair? There is something that does not convince me.
Is it true that the impulse of a movement does not matter?
So who is the the artist. Maybe also a scientist could be considered an artist: a scientist who finds a perfect algorithm...the idea..the concept, the beauty of a formula. Which is the difference in this case between the minimalistic beauty of a sculpure of Arp, and the simple beauty of Einstein formula, if The idea becomes the machine which makes art.
Well, the freedom of the observer.
Maybe the difference is in the potential interpretation. The scientist cannot leave space for observer interpretation. The artist can and does. From a pure sphere we can imagine the world, a ball, the belly of a mother...the observer is free to close his eyes and dream.
So maybe the execution is something more than a perfuncotry affair.
There must be a moment in which the the idea becomes art...and maybe it is in this moment in which the seed of immagination is dropped in the work.