I just finished reading the Bhagavad Gita, my first Hindu read. The concept of karma has captured my interest. Can one's thoughts contribute to their karma? For instance, if one thinks bad thoughts will bad things happen to them in this life or the next?
Thought is everything. Swami Vivekananda tells the following story (Complete Works, V8, Discourses on Jnana Yoga VI, available here - http://cwsv.belurmath.org/volume_8/vol_8_frame.htm):
Thought is all important, for "what we think we become". There was once a Sannyâsin, a holy man, who sat under a tree and taught the people. He drank milk, and ate only fruit, and made endless "Prânâyâmas", and felt himself to be very holy. In the same village lived an evil woman. Every day the Sannyasin went and warned her that her wickedness would lead her to hell. The poor woman, unable to change her method of life which was her only means of livelihood, was still much moved by the terrible future depicted by the Sannyasin. She wept and prayed to the Lord, begging Him to forgive her because she could not help herself. By and by both the holy man and the evil woman died. The angels came and bore her to heaven, while the demons claimed the soul of the Sannyasin. "Why is this!" he exclaimed, "have I not lived a most holy life, and preached holiness to everybody? Why should I be taken to hell while this wicked woman is taken to heaven?" "Because," answered the demons, "while she was forced to commit unholy acts, her mind was always fixed on the Lord and she sought deliverance, which has now come to her. But you, on the contrary, while you performed only holy acts, had your mind always fixed on the wickedness of others. You saw only sin, and thought only of sin, so now you have to go to that place where only sin is." The moral of the story is obvious: The outer life avails little. The heart must be pure and the pure heart sees only good, never evil. We should never try to be guardians of mankind, or to stand on a pedestal as saints reforming sinners. Let us rather purify ourselves, and the result must be that in so doing we shall help others.
Karmas are at three levels of body, speech and mind. So, yes, one's thoughts contribute to one's Karmas.
In Chapter 12, verse 5-8, Manu Smriti describes various kinds of sins (adharma/pApam) that a person commits. It classifies the sins into those committed through body, through speech and through mind.
It further says that the adharma committed through mind are- lusting after other’s wealth, wishing harm for others and adherence to falsehood.
So, karma is not just limited to the final actions but also to the words spoken and the thoughts that one indulges in.