Final answer:
William James, an American psychologist and philosopher, proposed that consciousness is a continuous stream of mental activity. His perspective contributed to functionalism and has greatly influenced modern psychology.
Step-by-step explanation:
The American psychologist and philosopher who proposed that the subjective experience of consciousness is an ongoing stream of mental activity was William James. James is known for his contributions to functionalism, a school of thought in psychology that emphasized the purpose of consciousness and behavior in helping individuals adapt to their environment.
William James differed from his contemporaries like Wilhelm Wundt in that James incorporated both introspection and more objective measures into his psychological research. His work laid the foundation for understanding consciousness not as a series of static elements, but as a fluid and continuous process.
He considered consciousness to be like a stream—ever-changing and flowing with an individual's mental activity. James's perspective was a departure from earlier psychological approaches and has had a lasting impact on the field, influencing subsequent theories on consciousness and cognition.