What did Renee Descartes mean when he stated " Be Careful What You Are Taught" ?

There is an indistinct line between education and indoctrination. A century after Descartes the 1500 year old doctrine of alchemy started to give way to modern chemistry. Millions of the most learned people were taught alchemy, and they were not careful enough.

> if anything?

Well, that's the problem. Either it's a massively simplified version of some Descartes quote, or completely made up - I can't find any sign of Descartes writing this.

His Meditations on First Philosophy began with the premise that all perceptions should be mistrusted, as part of a longer argument attempting to find a solid basis for what could be knowable. But that doesn't quite fit.

Every one gave such good answers ,opinions and insights. I am having a hard time Voting Best Answer

Be careful who you learn from, not all of those who teach are wise. And some are just wrong or immoral. You should not want to become what they are.