One of the most instructive of the older Upanishads is the Ch[=a]ndogya.  A sketch of its doctrines will give a clearer idea of Upanishad philosophy than a chapter of disconnected excerpts: 

All this (universe) is brahma.  Man has intelligent force (or will).  He, after death, will exist in accordance with his will in life.  This spirit in (my) heart is that mind-making, breath-bodied, light-formed, truth-thoughted, ether-spirited One, of whom are all works, all desires, all smells, and all tastes; who comprehends the universe, who speaks not and is not moved; smaller than a rice-corn, smaller than a mustard-seed, ... greater than earth, greater than heaven.  This (universal being) is my ego, spirit, and is brahma, force (absolute being).  After death I shall enter into him (3.14).[8] This all is breath (==spirit in 3.15.4).

After this epitome of pantheism follows a ritualistic bit: 

Man is sacrifice.  Four and twenty years are the morning libation; the next four and forty, the mid-day libation; the next eight and forty, the evening libation.  The son of Itar[=a], knowing this, lived one hundred and sixteen years.  He who knows this lives one hundred and sixteen years (3.16).

Then, for the abolition of all sacrifice, follows a chapter which explains that man may sacrifice symbolically, so that, for example, gifts to the priests (a necessary adjunct of a real sacrifice) here become penance, liberality, rectitude, non-injury, truth-speaking (ib. 17. 4).  There follows then the identification of brahma with mind, sun, breath, cardinal points, ether, etc, even puns being brought into requisition, Ka is Kha and Kha is Ka (4. 10. 5);[9] earth, fire, food, sun, water, stars, man, are brahma, and brahma is the man seen in the moon (4. 12.  I).  And now comes the identity of the impersonal brahma with the personal spirit.  The man seen in the eye is the spirit; this is the immortal, unfearing brahma (4. 15.  I = 8. 7. 4).  He that knows this goes after death to light, thence to day, thence to the light moon, thence to the season, thence to the year, thence to the sun, thence to the moon, thence to lightning; thus he becomes divine, and enters brahma.  They that go on this path of the gods that conducts to brahma do not return to human conditions (ib. 15. 6).

But the Father-god of the Br[=a]hmanas is still a temporary creator, and thus he appears now (ib. 17):  The Father-god brooded over[10] the worlds, and from them extracted essences, fire from earth, wind from air, sun from sky.  These three divinities (the triad, fire, wind, and sun) he brooded over, and from them extracted essences, the Rig Veda from fire, the Yajur Veda from wind, the S[=a]ma Veda from sun.  In the preceding the northern path of them that know the absolute (brahma) has been described, and it was said that they return no more to earth.  Now follows the southern path of them that only partly know brahma: