call a non static function in a static function in laravel 5

i am using laravel 5. And in a model i have a static function which i am calling in controller. It's working fine but i want same changes in this function with another non static function and when i am calling it inside static function it produce error.

Non-static method App\Models\Course::_check_existing_course() should not be called statically

Here is my model

namespace App\Models;
use Illuminate\Database\Eloquent\Model;

    class Course extends Model {
        public $course_list;
        protected $primaryKey = "id";
        public function questions(){
            return $this->belongsToMany('App\Models\Question','course_questions')->where("status",1)->orderBy("id","DESC");
        }

        public static function courses_list(){
            self::_check_existing_course();
        }
        private function _check_existing_course(){
            if(empty($this->course_list)){
                $this->course_list = self::where("status",1)->orderBy("course")->get();
            }
            return $this->course_list;
        }
    }

via Chebli Mohamed