https://github.com/databricks/scala-style-guide#private-fields

privateThisExplanation.scala

class Foo {
    private def isFoo = true
    def boo(other: Foo) {
        if (other.isFoo) { 
            // ...
        }
    }
}
class Foo {
    private[this] def isFoo = true
    def boo(other: Foo) {
        if (other.isFoo) {  // this line won't compile
            // ...
        }
    }
}