Which principle does Brentano emphasize about acts of consciousness?

Brentano's emphasis on acts of consciousness centers around the idea that these acts inherently direct attention toward objects. This notion is foundational in his philosophy of mind, as he posits that every mental act is characterized by intentionality, meaning that it is always about something or directed at an object. This focus on objects distinguishes consciousness from mere sensory experiences, underscoring the relational aspect of thoughts and perceptions. By recognizing that consciousness is always directed at an object, Brentano illustrates the active nature of mental processes, where mental states are not isolated occurrences but are fundamentally tied to the content they engage with.

The other principles do not accurately capture the essence of Brentano’s stance on consciousness. While considerations of subjectivity, measurement issues, and the relationship between consciousness and behavior are indeed relevant discussions in psychology, they do not encapsulate Brentano's distinctive focus on the intentional nature of mental acts. Thus, the correct answer highlights the core of Brentano's theoretical perspective in understanding consciousness.