I ought to have known that before ever I saw the box at all." This last statement appeared to me to be so startling, that I could hardly believe that he was in his sober senses.

There was the dead dog, however, to prove that his conjecture had been correct.

It seemed to me that the mists in my own mind were gradually clearing away, and I began to have a dim, vague perception of the truth.
"All this seems strange to you," continued Holmes, "because you failed at the beginning of the inquiry to grasp the importance of the single real clue which was presented to you.

I had the good fortune to seize upon that, and everything which has occurred since then has served to confirm my original supposition, and, indeed, was the logical sequence of it.

Hence things which have perplexed you and made the case more obscure, have served to enlighten me and to strengthen my conclusions.
It is a mistake to confound strangeness with mystery.