The firings of certain neurons in the brain that happens when you're thinking about something can be said to "exist" in the physical word. In this sense thoughts are a physical part of reality. That does not make them the same as something actually existing in reality, of course (i.e., me thinking about the Invisible Pink Unicorn - the firing of certain neurons in my brain when I think about Her Horniness - is different from an Invisible Pink Unicorn actually existing outside of my head). You can easily make a distinction: something existing outside of your head is objective, whereas you can think about whatever-the-hell nonsense you want. But if you wanted to completely pervert the word "exist," I guess you could make the argument.