However objective a historian may attempt to be, it is impossible for any work to be entirely without bias, if only because ideas about right and wrong (to take only one example) change so radically between eras and cultures. For instance, until the twentieth century, from the perspective of most British historians (and most British people who considered the matter), Sir Francis Drake was a great national hero, a man to be admired and emulated. From the Latin American perspective, he was a pirate, distinguished from other pirates only by the degree of his bloodthirstiness and appetite for destruction. If the British perspective has moved closer to the Latin American perspective in recent decades, this is due to a re-evaluation of imperialism rather than an obvious increase in objectivity.

Moreover, historians who are themselves writing from a particular perspective deal with sources which also reflect the perspective of the author. This perspective may be difficult to determine, particularly if many centuries or millennia have passed. For instance, Socrates was tried, among other things, for "not worshipping the gods of the state." Many historians have regarded this as a trumped up charge, asserting that hardly any educated Athenians at the time would have believed in the literal existence of Zeus, Athena, and the other Olympians. However, we cannot be sure of this, particularly since Socrates's own case shows that one could be tried for openly asserting disbelief in the gods.

Some historians have attempted partially to solve the difficulties around perspective by including various different perspectives in their work, as Simon Schama does in Dead Certainties. However, it is clearly not possible to account for all possible or even all actual perspectives in any work of history.

I always tell my students that history is always someone’s story. When reading a historical text, therefore, it is important to consider the historical perspective. Each historian brings a particular background, political view, and lens to any narrative. And those factors will shape the story that any historian tells.

Consider, for example, the history of the “discovery“ of the lands that would become America. Some textbooks present this narrative as Europeans reaching a vast wilderness and settling in to claim lands with an occasional skirmish with “savage” Native American groups. Other texts present a much different perspective, one that tells of the millions of people who already lived on these lands and who were victimized by these foreign groups who stole their lands.

Historical perspectives shape the story that is told. When analyzing any event of historical significance, it is important to consider various perspectives to discern the complex ways history has been written. Historical perspectives also allow us to determine common themes that run throughout history and continue to shape our world today.

History consists of objective facts, such as that the American Civil War lasted from 1861 to 1865, for example. But in order to interpret those facts, it's necessary to have some kind of perspective. In other words, if we're going to make sense of the past we need to approach historical events from a certain point of view.

Such standpoints could be overtly political, as in Marxist historiography, for instance. Or they could be more subtle, less politically engaged. But they can never be completely disinterested. Historians will always bring something of themselves—their background, their professional training, their cultural assumptions—to the study of history.

This does not necessarily mean that all historical perspectives are somehow distorted; some perspectives will always be more plausible than others, however partial they may be. But it does mean that absolute objective certainty in the interpretation of historical events is impossible, creating a large space for many different voices to offer competing narratives.

It should be noted that historical perspective can apply to the sources and material historians work with, because in order to make sense of a document, historians need to understand the mindset of the people who produced it. In many cases, the historical facts have been obscured by the perspectives of the people who have recorded them—whether they approve or disapprove of a given policy (let alone if they have a personal stake in it); whether they have a political agenda; whether they are trying to gain favor with ruling authority; whether they have strong moral, religious, or political convictions: those questions matter a great deal, and historians need to be able to answer them. The sources aren't unbiased, and historians need to be able to account for where and how far they can be trusted, or if they should be trusted at all.

Perspectives are important for all historians, because there is no such thing as objective history. This means that all historians have different ways to interpret events in the past. In other words, all events are known and told from the the biases of historians. We call his historiography. It is also important to add that all historians look at history with different interests in mind. Some historians examine intellectual history; others examine social history; still others look at another aspect of history. All of these are perspectives. 

An example might be helpful. 

When we come to the poems of Homer, some historians believe that the poems accurately reflect the Bronze Age. Other historians say that Homer's poems are completely made up and have no bearing on the Bronze Age. Most historians fall somewhere in the middle. The various views here are all perspectives on the same poem. This shows that differences of opinion exist. History, in this sense, is all about perspectives.