The word "Tathagata" designates all the qualities of the Buddha. We cannot grasp the Buddha’s wisdom. We can only acquire some understanding of it if we practise what he taught. We read in "As it was said" (Itivuttaka, Book of the Threes, Ch 5, Part 3) that the Buddha said:

“Monks, even if a monk should seize the hem of my garment and walk behind me step for step, yet if he be covetous in his desires, fierce in his longing, malevolent of heart, of mind corrupt, careless and unrestrained, not quieted but scatterbrained and uncontrolled in sense, that monk is far from me and I am far from him. What is the cause of that? Monks, that monk sees not Dhamma. Not seeing Dhamma he sees not me. Monks, even though a monk should dwell a hundred yoyanas away, yet if he be not covetous in his desires, not fierce in his longing, not malevolent of heart, not of mind corrupt, but with mindfulness set up and composed, calmed, one-pointed in mind and restrained in sense,— then indeed that one is near to me and I am near to him. What is the cause of that? Monks, that monk sees Dhamma. Seeing Dhamma he sees me.”