The leaders of China and Japan like to think of relations in terms of temperature. President Hu Jintao is optimistic his visit, starting today, will herald a 'warm spring' in ties. Japanese Prime Minister Yasuo Fukuda's predecessor, Shinzo Abe, saw his trip to Beijing in 2006 after five years of ill-will chill as 'ice-thawing'. Premier Wen Jiabao repaid the visit 13 months ago, seeing it as 'ice-melting'. No matter the measure of warmth, there is a single reality that the leaders of both nations cannot escape: they have to live with one another. As obvious as this may seem, it has too often been forgotten by the leadership of both sides. They have put history and rivalry ahead of the fact that they share the same patch of Asia. Neighbours, especially ones so dominant, cannot in such circumstances ignore one another; they have to work together. Businesspeople know this well. China last year became Japan's biggest trading partner. On China's trade books, Japan ranks third behind the European Union and US. Tourism figures on both sides are at record levels. The two are intrinsically entwined: ancient China was the source of Japan's language, art and much of its culture, while contemporary China's modernisation was inspired by Japanese success in emulating the west. Mr Fukuda realises the importance of co-operation. He has reached out to Beijing like no other Japanese leader, and Mr Hu and Mr Wen have taken his sincerity on board. There is a sticking point, though: Mr Fukuda is hanging onto power by a thread and his successor may not share his views. Japan was among the first nations to establish diplomatic ties with China after the end of the cultural revolution. But Japan's occupation of parts of China from 1931 to 1945 has stood in the way of friendship. Not until 1998 was a top-level effort made to mend fences, when then president Jiang Zemin became the first Chinese head of state to visit Japan. His expectation of a full apology for wartime aggression did not eventuate and the trip was considered a failure. Ties fell to lows under nationalist Japanese prime minister Junichiro Koizumi, whose obstinate visits to the Yasukuni Shrine to remember his country's war dead inflamed Chinese passions. There are a number of other contentious matters. The right to explore gas fields in the East China Sea, territorial disputes and the whitewashing of history by Japanese school textbooks rank high. Two others have emerged this year: Beijing's policy towards Tibet and the quality of food from the mainland. Mr Hu's trip was conceived months before January, when 10 Japanese became ill after eating contaminated dumplings, and confidence in food from China plummeted. Perceptions of China have been further soured by how Beijing dealt with protests in March in Tibet; repeated clashes between Chinese students and Japanese carrying Tibetan flags at the Beijing Olympics torch relay in the Japanese city of Nagano on April 26 did not help. Such matters are part of the ups and downs of relations between China and Japan. They will not take the shine from Mr Hu's trip or diminish the symbolic significance of his summit tomorrow with Mr Fukuda. No matter whether a major agreement is signed, the two sides are taking the important step of building solid foundations. Trust cannot be created overnight. History will not be wiped aside by statements. But nor can politics get in the way of moving China and Japan closer. They have no choice but to come to terms with their circumstances and, over time, become partners.