public function bind($key, $value)
    {
        if(is_callable($value)) {
            $this->aliases[$key] = $value;
        } elseif(is_object($value)) {
            $this->resolved[$key]  = $value;
        } else {
            if(class_exists($value)) $this->aliases[$key] =  $value;
            else throw new Exception("Class [ $value ] does not exist.");
        }
    }