User blog comment:Emlightened/Little Bigeddon/@comment-24061664-20170117003234/@comment-1605058-20170117083356

To complement Deedlit's reply with an example: note that \(=\) and \(\in\) are also binary predicates, so of course something like \(x\in y\) and \(x=z\) are not unary formulas. However, if we quantify one of the variables, for example \(\exists x:x\in y\). Now \(x\) is not a free variable and the formula only depends on \(y\), which makes it unary.