You plug your ears and watch a sparrow sing. You are sure that your watching a sparrow. Now, you close your eyes and listen to a sparrow sing. You can't be sure it's a sparrow, or a bird even. Your mind tells you to wait until you see it with your own eyes.
You would believe it was a sparrow if your eyes are open and your ears unplugged. You would believe it was a sparrow if your eyes are open and your ears plugged. You wouldn't believe it was a sparrow if your eyes are closed and your ears are open.
You can never be sure until you see it with your own eyes. Yet, what makes us so sure that our eyes are more accurate than our other senses, or our other perceptions such as emotions? We seem to think that our eyes assure us that something is material, that it exists as more than simply an idea, or a possibility. Who are we to think that any of our senses or perceptions can assure us of material things?
We cannot be sure. Devices of infared imaging, nightvision goggles, cybernetic implants, etc. cannot assure us either, for they are simply interfaces to our senses. Question everything.