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Below is an encrypt from John Lock's "An Essay on Human Understanding".

I am not able to understand the meaning of "without" in this context. The problem is that it's object is not mentioned. I viewed such an instance of "without" in Cambridge Dictionary, i.e,

"A long wool coat is a classic no wardrobe should be without."

But, here it is clear that it means "without it" i.e., "without a long wool coat". In other words, the object is removed here as it can be sensed from the context. But, I am not able to grasp what should be considered removed here?

The mind being every day informed, by the senses, of the alteration of
those simple ideas, it observes in things without; and taking notice
how one comes to an end, and ceases to be, and another begins to exist
which was not before; reflecting also on what passes within itself,
and observing a constant change of its ideas, sometimes by the
impression of outward objects on the senses, and sometimes by the
determination of its own choice; and concluding from what it has so
constantly observed to have been, that the like changes will for the
future be made in the same things, by like agents, and by the like
ways, considers in one thing the possibility of having any of its
simple ideas changed, and in another the possibility of making that
change; and so comes by that idea which we call power. (1690, Chapter
XXI, Section 1, pp. 219–220)

A clue is the later phrase within itself. You might guess from this phrase that the object of without is itself, and that without is the opposite of within.

And indeed, this is correct; here, without takes on its former meaning of outside. From Oxford Dictionaries Online:

2. 
   literary, archaic Outside. ‘the barbarians without the gates’
   

So what it means here is things outside the mind.

If you're reading things written in 1690, you have to watch out for words whose meanings have changed. The Cambridge Dictionary seems not to even mention this meaning, which I would guess means that it's not the best dictionary for looking up words from 1690. (In defense of the Cambridge Dictionary, this meaning is virtually never seen in today's English.)

I think you're looking at an old meaning of "without" here, namely, "outside" or "external". The only online reference I could find at a quick search was in the LEO English-German online dictionary:

without (obsolete, poetic): outside (preposition)

This makes sense, since Locke is talking about the distinction between the mind and the information it receives from the outside world - the "things without" - through the senses.

It's just the opposite of "within": "The mind... observes in things outside itself."

From the OED:

WITHOUT

A. adv. Outside, in various senses: opposed to WITHIN adv. Now only literary and somewhat arch[aic].

1. On the outside or outer surface (of a material thing); externally.

_{In this sense of the word, it's perfectly reasonable for your wardrobe to be without a long wool coat; what would be ridiculous would be to have your wardrobe within your long wool coat! 😀}