How to use "setTimeout" to invoke object itself

Why can't I use setTimeout in a javascript object?

Message = function () {

    ...
    ...        

    this.messageFactory = ...
    this.feedbackTag = document.getElementById('feedbackMessages');

    this.addInfo = function (message) {
        var info = this.messageFactory.createInfo(message); // create a div
        this.feedbackTag.appendChild(info);

        setTimeout('this.feedbackTag.removeChild(info)', 5000);
        // why in here, it complain this.feedbacktag is undefined ??????

    };
}

Thanks for Steve`s Solution, now it will work if the code is as below... because the 'this' before was actually pointing to the function within setTimeOut, it cannot rearch Message.

Message = function () {

    ...
    ...        

    this.messageFactory = ...
    this.feedbackTag = document.getElementById('feedbackMessages');

    this.addInfo = function (message) {
        var info = this.messageFactory.createInfo(message); // create a div
        this.feedbackTag.appendChild(info);

        var _this = this;
        setTimeout(function() { _this.feedbackTag.removeChild(info); }, 5000);

    };
}

But why doesn`t it work if we do this:

Message = function () {

    ...
    ...        

    this.messageFactory = ...
    this.feedbackTag = document.getElementById('feedbackMessages');
    // public function
    this.addInfo = function (message) {
        var info = this.messageFactory.createInfo(message); // create a div
        this.feedbackTag.appendChild(info);

        delayRemove(info);

    };
    // private function
    function delayRemove(obj) {
        var _this = this;
        setTimeout(function() { _this.feedbackTag.removeChild(info); }, 5000);
    }
}