I hope you don't mind if I cross-pollinate this board with some stuff from the E. But in this thread regarding whether thoughts really matter in terms of karma, Ven. Huifeng offers some very helpful insights (I sure learned from them), and they seemed worth posting here, too. So here they are:
Huifeng wrote:So, we have five aggregates:
1. physical form
2. sensation - pleasant, unpleasant, neither nor.
3. perceptions / conceptions - which are internal verbalizations
4. volition / formation - constructive acts, esp. including "cetana" and "karma", the various emotions (e-mote = to make move), etc.
5. cognition / consciousness - bare cognition, ethically neutral
Now, #1 is matter, so is out of our scope of "do thoughts really matter".
#2, #3 & #4 are called mental factors (cetasika)
#5 is mind itself (citta / mano / vinnana)
So, we are really only talking about #2~#5 inclusive for out topic here.
Mind itself (citta), doesn't really "think", and it is not really what we mean by "thought" (though old translations sometimes rendered it as such).
Sensations are ethically neutral, and, as Bhante Dhammanando states, they are more the result of karma. In this sense, passive.
So, this leaves us with perception / conception (sanna), and volition / formation (sankhara).
The early stages of perception are passive too, and ethically neutral. But, as they form into conceptions, they easily break into conceptual proliferation and views. These are types of defilements. However, there is some dispute as to whether "views" constitute karma or not. But, they can be seen as the basis behind karma. esp. the views "I", "mine" and "my soul".
Perception and conception in this sense are "thought" in the usual English meaning of the term, use words to think about stuff.
Volition / formation is definitely karma, by definition (see my earlier quote from AN). It is active, again by definition - the "-kara" in "sankhara" meaning "action", ("karaka" / "katr" = actor, etc.) They are emotions.
So, it may depend on what degree one considers emotions and volitions to be "thoughts".
Usually, in the Buddhist sense, they do not require words (unlike perceptions / conceptions), but the two run together. In most systems of buddhist psychology, in fact, all five aggregates always run together. However, this does not mean that they should be conflated together.
(Unless one is a strict Darstantika like Srilata, but lets not go there, and I doubt anybody will know who or what I am talking about! )
So, depending on your definition and range / scope of the word "thought", hopefully this should provide some basis for answering the OP:
"Do thoughts really matter in terms of karma?"