We know there is an external world outside/beyond ourselves because of the following reaasons:

1. We know we have perceptual senses of sight, hearing, touch, smell, and taste because we are aware of seeing, hearing, touching, smelling, and tasting because we cannot deny or otherwise turn off our perceptual senses.

2. As we see, hear, touch, smell, and taste, we develop memories of seeing, hearing, touching, smelling, and tasting Xs we can call entities, we become aware and thereby learn that we cannot deny the reality/existence of these entities, and as we gain experience with entities we begin to differentiate them into people and objects and we become aware and thereby learn of events as relationships between or among entities (event = causal or coincidental relationship inre people or/and objects).

3. We become aware and thereby learn of space as distance between entities and event or the distance-relevant dimensions of entities and events including length, width and depth.

4. We become aware and thereby learn of time as duration between multiple events, duratios of single events, and duration (ages) of entities (people and objects) and space.

5. We become aware and thereby learn of the location or positions of entities and events within space and time; we use spatial coordinates x, y and z to locate an entity or event from an origin in space and the temporal dimension t to locate an entity or event from an origin in time, and we become aware and thereby learn of the relationships between space, time and motion which are d = rt, r = d/t, and t = d/r ...

where ...

d = distance;
r = rate (speed, velocity when both speed and direction are combined);
t = time.

6. We become aware and thereby learn of mass as the matter and energy which comprise all entities and events which are all phenomena which are neither space/distance nor time/duration, and we become aware and thereby learn that we are comprised of matter-energy and therefore mass and we exist in space and endure in time, and thereby we become aware and learn of the universe as the combination of space (distance), time (duration) and mass (matter-energy): universe = space + time + mass (matter-energy).

7. We experience our bodies by our awareness of our organic sensations as well as our perceptual senses of sight, hearing, touch, smell, and taste; we become aware and thereby learn of our personal dimensions of size and weight, we become aware and thereby learn that from within ourseslves we can cause ourselves to move, to act, and to react, by having desires for people, objects and events (desire = wanting a person, object or event), and fears of people, objects and events (fear = not-wanting a person, object or event), and priorities among those desires (priority = the importance of each desire and fear relative to all other desires and fears), and we bcome aware and thereby learn that our desires, fears and priorities comprise our minds (mind = individual's personal set of desires, fears and priorities which cause our behavior as our actions and reactions, our feelings (sensations and emotions) as our reactions to realizations (realization = achievement or non-achievement of a desire—of a desired person, object or event, or avoidance or non-avoidance of a fear—of a feared person, object or event) of our desires and fears (feelings = reactions to realizations of desires and fears), our personalities as our consistent actions and reactions in similar situations, our mental problems (mental disorders) as our unrealistic (unachievable and/or inappropriate) desires (mental problems/disorders = un-peace-of-mind), and our mental health as our realistic (achievable and appropriate) desires (mental health = peace-of-mind), and we become aware and thereby learn of our physical boundaries and physical and mental limitations.

8. We become aware and thereby learn that due to our personal dimensions and boundaries and limitations that we cannot occupy the space coordinates occupied by another entity at the time coordinate at which the other entity is present. We become aware and thereby learn that we cannot occupy the space coordinates of a brick wall at the time coordinates at which the brick wall is present.

9. Thus, by our awareness and learning of entities and events and space and time, we become aware and thereby learn that there is an external world which we cannot deny or evade.

If you do not believe in any of the above, try occupying the space and time coordinates of a brick wall.