Both examples are of reported speech. Yesterday you said
"I can solve puzzles."
In the 1400s, Leonardo said
"Defective vision can be corrected by placing a lens in direct contact
with the eye."
Thereby anticipating the contact lens industry by centuries. When we report these words later without quotes, we have to correct, where required, for person, place, and tense.
For tense, reporting may be "back shifted" to the past. You actually said (perhaps to yourself) the words "I can," but that was yesterday, which you may indicate by using the past tense:
Yesterday, I realized that I could solve puzzles.
Notice that I said that you "may" back shift. It's not always required, and for things that continue to be true in the present, and especially those things that always remain true, you may choose to keep the verb in the present tense. Thus,
Yesterday, I realized that I can solve puzzles.
means that you could solve puzzles yesterday when you realized that you had that talent and that you can still solve puzzles today.
Likewise for Leonardo's statement. The laws of optics haven't changed since Leonardo's time and they're not like to change ever, and this is implied by the statement:
In the fifteenth century, Leonardo da Vinci suggested that defective
vision can be corrected by placing a lens in direct contact with the
Contact lenses would have worked in Leonardo's time, they work today, and they will continue to do.