OUR KNOWLEDGE OF EXTERNAL OBJECTS ONLY SENSATION Modern theories of matter--Outer world only known to us by our sensations--Instances--Mill's approval of proposition, and its defects--Nervous system only intermediary between self and outer world--The great X of Matter--Nervous system does not give us true image--Mueller's law of specificity of the nerves--The nervous system itself a sensation--Relations of sensation with the unknowable the affair of metaphysics.