In Critique of Pure Reason, what does Kant mean by the statement "Thoughts without content are empty, intuitions without concepts are blind?"
This famous quotation refers to the necessary correlation between abstract thought and concrete reality. In context, the quote refers to the idea that the mind can only process what is given to it by the senses, but it also can create intuition, or instinct, which is not based in sensory reality. Here is the quote in context:
Our nature is so constituted that intuition with us never can be other than sensuous, that is, it contains only the mode in which we are affected by objects. On the other hand, the faculty of thinking the object of sensuous intuition is the understanding. Neither of these faculties has a preference over the other. Without the sensuous faculty no object would be given to us, and without the understanding no object would be thought. Thoughts without content are void; intuitions without conceptions, blind.
(Kant, Critique of Pure Reason, gutenberg.org)
The mind produces abstract representations of the sensory input it receives; this is called "understanding" because the mind must interpret what it experiences without any other input. When thinking about something, the thought is bonded to memories of sensation, not of rational reason; the sensations produce both memory and action, and are required for the mind to navigate the physical world.
"Thought" is the mind working through its sensory input, and usually requires some sort of "content" to operate; there is abstract thought, which in Kant's view is empty because it is "motion" of thought without any larger purpose. Even thinking about dinner causes choice, emotion, or just hunger; "empty" thoughts have no larger purpose.
Intuition is based in the mind focusing on some single aspect of its sensory input, and acts as an unconscious nudge towards some decision or goal; however, the intuition must be informed by "conception," or real experience, as the creation of context for daily sensory input. Without these conceptions, the intuition is "blind" because it has no context to become meaningful.
This statement has to do with people's ability to comprehend the world around them: in order to understand their surroundings, people rely both on their perception and on a pre-existing foundation of knowledge. Without this a priori "content" or "concept," people cannot make sense of the things they experience.