Now what was interesting about the Incas is that they were able to form this large empire, the largest empire in the Americas at the time of the European conquest. This empire had 10 million people in it, and what's amazing is that you have this large, powerful empire with significant building projects. This is a picture of Machu Picchu. Historians believe that it was built as an estate for Pachacuti, so they were able to do the sophisticated construction despite not having a written system as we know it. They had a system of knots for some forms of record-keeping, but despite that, they were able to have a sophisticated society. Their way of taxing people was not through formal coinage. They didn't have a monetary system as we know it.