Brentano, a 19th-century philosopher, argued that intentionality is the defining characteristic of all mental phenomena. He believed that every conscious act is directed towards an object, whether it is a physical object, an abstract concept, or even a feeling.
While Brentano himself didn't use the exact phrase "consciousness is intentional," his work laid the foundation for this idea.
Other philosophers, like Edmund Husserl, further developed this concept and made it central to phenomenology, a school of philosophy that focuses on the subjective experience of consciousness.
It's important to note that not all philosophers agree with Brentano's claim. Some argue that consciousness can exist without being intentional, while others believe that intentionality is not the defining characteristic of consciousness. However, Brentano's work has been very influential, and the idea that consciousness is intentional remains a central topic of discussion in philosophy of mind.