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...laced by modern formal logic, which has its roots in the work of late 19th-century mathematicians such as [[Gottlob Frege]]. Today, the most commonly used sys ...roduction: Philosophy of logic today, pp. 1–12}} Starting in the late 19th century, many new formal systems have been proposed. There are disagreements about ...

...the human population will begin to decline in the second half of the 21st century.<ref>{{Cite news |last1=Cave |first1=Damien |last2=Bubola |first2=Emma |las ...s, broken down by continents, were published in the first half of the 19th century, at 600 million to 1&nbsp;billion in the early 1800s and 800&nbsp;million t ...

==Recorded history before 12th century== ...Oxford|access-date=14 October 2014|url=http://www.visitoruk.com/Oxford/8th-century-T4311.html}}</ref> ...