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I couldn't understand the bold lines. Please help.
Whig history or Whiggish historiography presents the past as an inevitable progression towards ever greater liberty and enlightenment, culminating in modern forms of liberal democracy and constitutional monarchy. In general, Whig historians stress the rise of constitutional government, personal freedoms and scientific progress. The term is often applied generally (and pejoratively) to histories that present the past as the inexorable march of progress. It also refers to a specific set of British historians. Its antithesis can be seen in certain kinds of cultural pessimism.
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The term is often applied generally (and pejoratively) to histories that present the past as the inexorable march of progress. -
This term is often applied generally (and in a such a way as to lessen the value) to histories that show the past as the relentless/incessant march of progress.

Its antithesis can be seen in certain kinds of cultural pessimism
(Its) The direct opposite can be seen in certain kinds of cultural pessimism (opposite of optimism)
Hope that helps some
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Perhaps an exaggeratedly optimistic view of history.