Some of the most powerful Seer's don't need to use their eyes to see.
There are two kinds of Seers. One kind uses a form of spellcraft to gain knowledge of other places and times. Some users of this form are naturally gifted with visions or prophetic dreams. Others still may require a small incantation to gain the ability. No matter which way the Seer learns with enough practice it would be impossible to tell the difference.
The second kind of Seer is one that uses tools to gain their knowledge. Some of the most common tools used by these Seers are Tarot cards, or reflective items like water or a crystal. There are literally thousands of forms of this type of Seer all with their own tools and practices. This kind of Seer rarely makes it to the same level as the first, but can be just as accurate if not more in their readings.