Sherlock Holmes expresses a significant idea about the nature of knowledge and certainty in his quote. He acknowledges that while one may possess an undeniable understanding of a truth, articulating or justifying that knowledge can be challenging. This reflects a common human experience where intuition or instinctively known facts do not always lend themselves to logical explanation or proof.

The example of proving that two plus two equals four highlights the struggle between knowing something fundamentally and the complexities involved in demonstrating that knowledge. It suggests that certainty can exist independently of rational explanation, emphasizing the intricacies of human cognition and insight.