Pagination Not Working.

<?php

public function listCourses(Request $r)
    {
        $sort               = $r->get('sort');
        $category_id        = $r->get('category_id');
        $keyword            = $r->input('filter');
        $courses            = $this->course_discovery->paginate(10);
        //PAGINATION NOT WORKING

        $courses            = $this->course_discovery->listCourses();

        $sortOrder = $sort == 'a' ? 'asc' : 'desc';


        $filter = $this->course_discovery
                    ->where('category_id', 'like','%'.$keyword. "%{$category_id}%")
                    ->orWhere('category_id', 'like', "%{$category_id}%")
                    ->get();

        if ($sort == 'a' OR $sort == 'd') {
            $courses = $this->course_discovery
                ->select('course.course_id', 'course.course_title', 'course.course_overview', 'category.category_name', 'course_level.course_difficulty')
                ->join('category', 'course.category_id', '=', 'category.category_id')
                ->join('course_level', 'course.course_level_id', '=', 'course_level.course_level_id')
                ->orderBy('category.category_name', $sortOrder)
                ->orderBy('course.course_title', $sortOrder)
                ->get();
        }  else {
            $courses = $this->course_discovery->listCourses();
        }
        return response()->json([
            'Result' => $courses
        ]);
    }