Gerund: When "thinking" is used as a verb form ending in "-ing" and functioning as a noun, it's a gerund. For example:
* "Thinking about the future can be stressful." (Here, "thinking" is the subject of the sentence.)
* "I enjoy thinking about complex problems." (Here, "thinking" is the object of the verb "enjoy.")
Abstract noun: When "thinking" refers to the general concept of the mental process itself, it's an abstract noun. For example:
* "Thinking is a complex cognitive process." (Here, "thinking" is the subject of the sentence and refers to the act of thinking in general.)
* "She has a lot of thinking to do." (Here, "thinking" refers to the amount of mental effort required.)
So, the classification of "thinking" depends on its specific use in the sentence.