You're familiar with the Placebo Effect, but probably not with the Nocebo Effect.

The Placebo Effect states that if you believe a medicine will have a positive effect, it is more likely to - even if it is an otherwise "inert" substance.

The Nocebo Effect is the opposite - if you believe it will harm you, it is more likely to.

It sounds pretty profound and it is.

But it's not that simple.

If our intentions were the sole determiner of our outcomes we wouldn't have such things as "The Law of Unintended Outcomes."

It's not a formal scientific law per se, but it refers to the phenomenon whereby most everything we do has at least some unintended consequences.

My life in military intelligence tells me this is very true.  Almost every "op" has some "blowback" - that is, an unintended "side effect" of an operation that is intended to render a positive outcome.  (For example, the CIA's Operation Ajax that overthrew the democratically elected Dr. Mohammed Mossadeq and installed the US-friendly autocratic Shah.  But ultimately that seems to have resulted in the not-so-US-friendly Islamic Revolution and the famous Iran Hostage Crisis.)

Drugs have side effects that can not be predicted by the makers or the takers of medicine.

Sometimes we can observe a correlation between gun control and an increase in crime.

So, our intentions seem to have some effect on desired outcomes, but sometimes things happen that we never intended at all.

How much power does intention have, then?

We really have no idea, and we're not doing ourselves any favors by pretending we do (despite what some pseudo-scientists will have us believe).

One thing seems sure, though.  If you intend for something to happen and take deliberate action to make it so, you're surely much more likely to make it happen than if you had not.

Was it the intention or the action?

Well, try flipping on a light switch with the intention "it won't work." That might give you your answer.