Especially religious ideas.
If an idea can be demonstrated in some way- by logic, weight of evidence etc- it may not matter who's voicing it.
But if it's one of those ideas that comes out of a clear blue sky accompanied by angel voices- promising fulfilment, enlightenment, God's favour or whatever- I think we need to ask, "says who?"
And the follow up question- once we've ascertained the identity of the prophet- is, "what 's in it for him?"
Because the unfortunate fact is that religious prophets- of every stripe- tend to be lying, abusive, power-hungry SOBs.
I'm not saying they all are- just most of them.
It may be true- as michaleen argues- that Carlos Castenada's spiritual system stands apart from the man who invented it.
After all, that was my view of Wicca. Gerald Gardner was a mischievous old chap with a taste for nudism and masochism who invented a religion to service his needs- but it escaped from him and became something bigger and better and more interesting than he'd envisaged.
Knowing the history, I could make the ideas my own. And play with them. Just as Gardner himself did.
So, by all means develop a spirituality based upon a reading of Castaneda- but, unless you want to be flying blind, be aware that the man was a liar, a con-man and a total shit.
Or- to use Biblical language- "test the spirits".
And really we should be doing that with the mainstream religions as well. Muhammed was a middle-eastern warlord who massacred populations and ran a harem. Jesus was- well- Jesus doesn't actually show up in the historical record and may be a fictional construct.
Even the best of prophets is conditioned by time and place and culture.
Accept nothing on authority. Know where the teacher is coming from. Take back the power.