Skepticism is a classic topic in philosophical inquiry. In contemporary analytic philosophy, scenarios like "brains in a vat" manipulated by supercomputers are often used as paradigmatic thought experiments to construct and debate skeptical arguments. Developments in modern technologies such as virtual reality provide a serious and tangible foundation for reflecting on these once science-fictional "brains in a vat" scenarios. This lecture will revisit classic philosophical analyses of the "brains in a vat" scenario, clarify the types and essence of skeptical problems, and evaluate proposed solutions (or deflationary therapies) such as semantic externalism and bit realism. The discussion will ultimately lead to philosophical reflections on the contentful-ness of virtual reality systems or artificial intelligence systems, thereby illustrating the intersection between classical philosophical issues and contemporary philosophy as conceptual engineering.