Warren Buffett makes investment decisions based on business fundamentals rather than the broader market environment.The S&P 500 (^GSPC 0.48%) has advanced 12% year to date, notching 25 record highs in the process. What makes that number particularly extraordinary is the index was actually down 19% from its high in April when President Trump announced tariffs. But it quickly rebounded and has since rocketed 32% from its April low.That upward momentum begs the question: Should you really buy stocks with the S ...