There has to be an appeal for the umpire to give a batsman out.
In cases like being bowled, technically, the batsman walks. ( I suppose batsmen can always give themselves out ).
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The rules!!!
27.1 Umpire not to give batsman out without an appeal
Neither umpire shall give a batsman out, even though he may be out under the Laws, unless appealed to by the fielding side. This shall not debar a batsman who is out under any of the Laws from leaving his wicket without an appeal having been made. Note, however, the provisions of 7 below.
27.2 2. Batsman dismissed
A batsman is dismissed if
either (a) he is given out by an umpire, on appeal
or (b) he is out under any of the Laws and leaves his wicket as in 1 above.
27.7 Batsman leaving his wicket under a misapprehension
An umpire shall intervene if satisfied that a batsman, not having been given out, has left his wicket under a misapprehension that he is out. The umpire intervening shall call and signal Dead ball to prevent any further action by the fielding side and shall recall the batsman.
27.9 Umpire's decision
An umpire may alter his decision provided that such alteration is made promptly. This apart, an umpire's decision, once made, is final.
I like 27.2 and 27.9 because they declare what so many people on this board fail to realise...That the umpire making his decision is part of the rules and is therefore no less a part of 'is a batsman out?' than did it pitch in line.