How should I program with dynamic typing language even no type is noted on documentation?

Question

I lived long time on static typed language world. All types were explicitly noted on each variables and arguments. Now I have to use some kind of dynamic typing language. (Smalltalk) The problem is there is no type noted on any message argument, and even on documentation. So, how can I know what should be passed to the each argument? Why the people didn't note type on message arguments? Really isn't it need?