We spend our days perceiving the world around us. We see the sky, hear traffic, and feel the breeze. These are our external senses at work, gathering information from outside our bodies.

But we also have a way of perceiving what happens inside our minds. This is our inner sense. It’s the faculty that allows us to be aware of our own mental activities. When you notice you're feeling happy, thinking about a problem, or remembering a past event, you are using your inner sense. It's the foundation of self-awareness.

The key difference is the object of perception. External senses perceive physical objects and events in the world. The inner sense perceives the operations of our own minds, such as thinking, doubting, believing, and feeling.

The idea of an inner sense isn't new. The philosopher John Locke was one of the first to describe it in detail. He called it an "internal Sense" and argued that it was just as important as our external senses for gaining knowledge.

Locke believed the mind starts as a blank slate, or tabula rasa. He proposed two sources for all our ideas: sensation (from the external senses) and reflection (from the inner sense). Sensation gives us ideas about the outside world, like 'yellow' or 'hot'. Reflection, or the inner sense, gives us ideas by observing our own mental operations, leading to concepts like 'thinking' or 'willing'.

For Locke, the inner sense wasn't a mysterious or spiritual faculty. It was simply a natural capacity of the mind to turn its attention upon itself. Just as we can choose to look at a tree, we can choose to reflect on the fact that we are thinking about that tree. This simple act of turning awareness inward is the core of what he meant by the internal sense.